Thank you Maximilian for your help!
I found more information in another post, and I don't know why they haven't included this information in the Cross-Compiling for the Omega documentation.
José Luis Cánovas included these instructions in his Docker SDK post:
You must add the onion feeds (work in progress for my docker image, in the meantime do it manually) to the file feeds.conf.default (check WereCatf's first_time_setup.sh), update the feed and add the onion packages:
scripts/feeds update -a
scripts/feeds install -a
make menuconfigselect the onion libraries (Onion -> Libraries/Utilities -> select as [M]odule).
makehere to compile the toolchain]
ls staging_dir/target-mipsel_24kc_musl/usr/lib/(note the penultimate
omega@dee42fad6a75:/lede$ ls staging_dir/target-mipsel_24kc_musl/usr/lib/ [...] liboniondebug.so [...] libonioni2c.so [...] libonionspi.so [...]
and many more onion and non-onion libraries.
And inside staging_dir/target-mipsel_24kc_musl/usr/include/:
omega@dee42fad6a75:/lede$ ls staging_dir/target-mipsel_24kc_musl/usr/include/ [...] onion-spi.h [...] onion-debug.h [...] onion-i2c.h [...]
Note: I didn't need to add the Onion feeds to the
feeds.conf file - the
scripts/feeds script worked without any modifications.
I wish it were a little more clear about which libraries, etc. to select on the
make menuconfig, because I found multiple items that were related to I2C. So I just picked the ones that looked right...hopefully that doesn't mess me up too much.