Buildroot
Buildroot is a set of scripts that generates tiny distros with rootfs images smaller than 10MiB.
It downloads everything from source and compiles it, so it is trivial to patch packages up.
The generated images are so tiny, that it becomes possible to understand the entire userland setup, which will make it easier to focus on the kernel.
Advantage over LFS: everything is fully automated. Because of this, Buildroot is used professionally in large organizations.
I have created this setup to automate things as much as possible: https://github.com/cirosantilli/linux-kernel-module-cheat
