Aboriginal Linux is a shell script that builds the smallest/simplest linux system capable of rebuilding itself from source code. This currently requires seven packages: linux, busybox, uClibc, binutils, gcc, make, and bash. The results are packaged into a system image with shell scripts to boot it under QEMU. (It works fine on real hardware too.)
Move from busybox, uclibc, and gcc/binutils to toybox, musl, and llvm (then qcc).