lisa_shell: add a new function to generate buildroot rootfs

Created by: qais-yousef

buildroot rootfs are a better choice to run lisa tests on as they are quieter and provide a minimal userspace that allows testing kernel functionality with minimum interference from userland. Also it is less of a maintanence burden as it's unlikely to find unexpected things configured without our knowledge that might intefere with some of the tests or skew their results.

A recent example was found in systemd indirectly causing higher number of kthreads to run on debian and arch-linux distro's which caused random number of failure rates in cpu migration load tracking tests.

Signed-off-by: Qais Yousef qais.yousef@arm.com