linuxboot: add build script for linuxboot test

Add a new build script which builds the rdinfra platform software stack
for linuxboot test. This script also builds the stage-1 linuxboot kernel
and prepares a disk image with stage-2 linux kernel and busybox ramdisk.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I7fff7fbcc5d331a248c87c066a4b29bac56d086e