sgi/build-test-sct: add a build script for UEFI SCT test on sgi platforms

Add a new build script to allow building the SGI stack for UEFI SCT test.
This script is a wrapper on the build-all script. After completing the
build of the required components, this script prepares a disk image with
EFI parition and UEFI SCT pre-installed in it.

Change-Id: Iaa88ffa5f063d2a301babd1f376572c5aa82f85c
Signed-off-by: Lokesh B V <lokesh.bv@arm.com>