build-sgiboot: add a new build script for busybox boot on sgi platforms

Add a new build script to allow building the SGI stack for busybox boot.
This script is a wrapper on the build-all script. After completing the
build of the required components, this script prepares a boot disk
image with EFI and ext3 paritions in it.

Change-Id: I1302e72284fc8539b1a306e49c52f18815b0bd4d
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>