@Armstead-Smith if the individual app is a package in a feed (can be a third party feed like Onion's, or your own), you can build it separately with
This will produce a *.opkg package in bin/packages/mipsel_24kc/FEEDNAME. This can then be copied to the omega2 and installed with opkg.
Setting up your own feed with your own packages is not entirely trivial, but not really difficult either (with some time invested in reading up). I think it's worth doing, to get a nicely organized build.
If all you need is cross-compiling a binary without packaging, you could probably directly use the tools from the toolchain LEDE has build for you in build_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl-1.1.16. You'd need a bunch of environment settings to direct compiler and linker to the correct headers and libraries - but I don't have practical experience with that.
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