smaller images

[nickdesaulniers]

Speaking with @groeck , he pointed out some configs for buildroot that he uses to keep images small.

example:
https://github.com/groeck/buildroot/blob/local-2023.02/configs/kerneltests_aarch64_defconfig

So Guenter uses local-* branches, then puts the configs under configs/kerneltests_. I think these images have more configs enabled to enable kselftests, so we might not need them all.

In particular, Guenter mentioned switching to musl was a size win, but recommended avoiding a particular version:

don't use musl with version 2023.2.