[TESTS] arm32: Add busybox-1.21.1 defconfig and update documentation

We quite often get questions regarding cross-compiler to be used for
compiling Xvisor, Guest Linux kernel, and Guest RootFS.

The file <xvisor_source>/docs/SupportedToolChains only talks about
cross-compiler to be used for Xvisor and Guest Linux kernel.

This patch adds info regarding cross-compiler to be used for compiling
BusyBox based rootfs and busybox-1.21.1 defconfig.

Signed-off-by: Anup Patel <anup@brainfault.org>