you will need to have a mechanism to get a new image in without a working one - likely either a USB drive to U-Boot, TFTP over wired ethernet to U-Boot, or less likely directly programming the SPI flash chip with someone else or using JTAG to load in the code and then U-Boot to write it.
I am completely unfamiliar with those techniques. Do you have any references on any of them handy?
@Chris-Stratton
I have the OpenWRT 15.05 source tree. But when I run make menuconfig, and look at the options for Firmware, I'm presented with choices for everything from ar3k-firmware to wl18xx-firmware, none of which are selected, none of which I know whether to select or not. There's an even larger plethora of hardware bits to select from in the kernel modules. For instance, even if I don't plan on using it, I'm fairly confident that a stock Omega2+ has the kmod-ledtrig-morse kernel module installed, but it's not highlit by default in menuconfig; more seriously, not one of the kernel modules for network devices or wireless drivers is already chosen.
My worry is that if I don't pick exactly the right bits of hardware here, then when I try to use my home-compiled OS on my Omega2+, I'll have left out an important piece that will prevent me from even connecting to the Omega2+ again, effectively bricking the thing.
I'm hoping that if someone already has a .config that works, then I can safely use that as a guide to which modules for hardware support I need, and I can build from there. Failing that, maybe a simple list of which bits of hardware are on the Omega2+ (as opposed to the Omega) that need such software to be compiled would be enough to keep me from reproducing my earlier mistake of trying to compile for entirely the wrong board. Failing /that/... maybe I could try compiling a monolithic kernel that just includes support for everything, but that's hardly optimal.
Rephrased: My current problem is a lack of info on what chips and such are on the Omega2+ that need kernel/OS software compiled for them.
I find myself nervous about selecting the right hardware options in make menuconfig.
Has anyone got a .config file for openwrt which you already know works with an Omega2+, which you'd be willing to share?
I'd recommend against using an SD card as a root filesystem if you can at all avoid it.
May I ask why? I'm already using it as an overlay, and my plan is for most disk activity to be on the multi-terabyte BTRFS drive that's the whole point of this exercise.
you can also build a kernel with the filesystem packed as an initramfs that will uncompress and run from RAM
I suspect I'm going to have trouble enough just figuring out which kernel modules I'm going to need to include to support the Omega2+'s built-in hardware, so that I don't accidentally brick the thing. (I'm tempted to use make menuconfig and just select /all/ the blank items to compile as modules to be on the safe side...)
Honestly, I find myself amused at the contortions I'm having to go through to accomplish the goal of "stock Omega2+ or better, plus support for BtrFS". At this stage, if anyone here happens to have something I can download that includes that, I'm willing to forgo the current character-building learning experience.
@José-Luis-Cánovas
I think I've got the settings to compile the openwrt packages; if I can't figure out how to get them to work, I'll give LEDE a try.
@Chris-Stratton
Ah, thank you, there we go: "Target System: Ralink RT288x/RT3xxx", "Subtarget: MT7688 based boards".
With that setting, I've now compiled https://www.datapacrat.com/temp/kmod-fs-btrfs_3.18.45-1_ramips_24kec.ipk ...
... and opkg is giving me the same errors it did before. Phooey; it looks like I won't be able to take the shortcut of just compiling the one module I need, I'll need to compile the whole set of packages. Looks like I've got a weekend project of looking up how to use the Omega2+'s built-in OS as a bootloader, and installing the actual OS onto my 32 GB microSD card. (I know I saw the instructions for that somewhere, I've just lost track of where.)
@Chris-Stratton
The menuconfig I have is currently set to 'Atheros AR7xxx/AR9xxx'. ... I don't see any options resembling either 'ramips' or 'mt7688'. Am I looking in the wrong place? (I grabbed the openwrt files on Feb 4th, so going by my web history, it's the 15.05 version from http://git.openwrt.org/?p=15.05/openwrt.git;a=summary .)
And of course it couldn't be that easy.
I'm trying to compile a module for BtrFS, for my Omega2+; so far, the result of my efforts is this file: http://www.datapacrat.com/temp/kmod-fs-btrfs_3.18.45-1_ar71xx.ipk
Unfortunately, when I bring that file onto my Omega, and try to use opkg to install it, I get an error. (And of course Putty doesn't seem to be letting me copy the error's text directly - yes, I know Putty is designed to use the mouse for that.) I run opkg install ./kmod-blah, and get...
Unknown package 'kmod-fs-btrfs'.
Collected errors:
... I have a suspicion that the error arises because the compilation instructions I've been working on were based on the first Omega, and need tweaking for the Omega2+. So, new question: When compiling for the Omega2+, what's different than when compiling for the original Omega?
... Hunh. I started to run make in the openwrt directory, to generate the error messages to post here... but it kept going past where it stopped before, and after having let it run a while, it looks like it's actually successfully compiled.
The only possibly relevant changes to the system I can think of are that I installed the packages for Docker before I realized they were obsolete, including all the dependencies; and when I uninstalled the main Docker packages, the dependencies didn't go away. So maybe they were what my system needed. Or maybe my system just needed to try having make run a few times in a row to get past whatever the problem was.
In short: Looks like this thread is now unnecessary. (Unless something else turns up as I try to compile some particular thing.)
I saw the Docker thread, and even tried it, but the laptop in question is stuck with Red Hat 23 until I can figure out how to handle the partitioning system in Red Hat's upgrade process, and the Docker install instructions require RH24 or higher.
I have an idea or two for my Omega2+, if it has a consistent address I can access it at. Installing a Dynamic DNS client would do the trick; has anyone managed to get that working?
(An alternative would be to set up a "hidden service" on Tor, which would provide a consistent .onion address - has anyone managed this, instead?)
I've tried to follow the relevant instructions, but Red Hat uses somewhat different package names than are suggested for Debian-style Linuxes. I've had no trouble using make to compile and install other bits of software, but when I try to 'make tools', the effort fails partway through.
Am I going to need to install a VM of a Debian-style OS, or something? Or is there an easier way?
I just might be able to figure this one out with a lot of trial and error, but I'm hoping the good people here can save me a good deal of effort. 
I want to be able to set up my Omega2+ so that, once an hour, it creates a directory named for the date-and-time, cd's into it, and then runs wget to download local copies of a few hundred short files. (RSS feeds, if you're curious). I /almost/ know enough scripting to be able to do this on a Linux laptop; but the Omega's limited toolset makes it just enough trickier that I'd appreciate some help sorting out the details.
Gmail allows you to download all the messages in your account via either POP or IMAP. I'd like to configure my Onion Omega2+ to automatically do exactly that, onto the microSD card I'm going to buy today, so that I've got a local copy updated as long as my internet connection is working. Any advice on getting POP to work with an Omega?
Have myself an Onion Omega2+; and I've got a 4-terabyte hard-drive, formatted to BTRFS, which connects via USB. I want to access this drive with a laptop in another room. Reformatting isn't really an option.
If the drive were ext4, that would be easy; I could just follow the directions at https://wiki.onion.io/Tutorials/Using-USB-Storage to mount it, and https://wiki.onion.io/Tutorials/Sharing-with-Samba to share it.
What do I have to do to teach my Omega to talk to this hard-drive? Is there a compatible opkg package like the kmod-fs-btrfs one at https://wiki.openwrt.org/doc/howto/storage, do I need to compile a custom kernel, or what?
Thank you in advance for your help.