linuxboot: add new script for validating linuxboot

Add a run script for testing linuxboot on RD Platforms. This script
launches the FVP with an attached satadisk image that contains the
stage-2 linux kernel and busybox ramdisk image.

This is an initial implementation of linuxboot for RD Platforms and not
the final version.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I7209ae2fa1a99fc670f281eafedf2a9791c0fd09