[DOCS] Remove refrences to zImage from ARM host READMEs

Some of the README files under docs/arm still contained reference
to zImage booting and build zImage for Guest linux.

This patch fixes this README files.

Reported-by: Anshul Makkar <anshulmakkar@gmail.com>
Signed-off-by: Anup Patel <anup@brainfault.org>