sgi/build-test-secureboot: add build support for validating secure boot

Add support for building the fip and disk images for validating secure
boot functionality on Arm's SGI platforms. The build packages busybox
as the filesystem and places all the required signatures into the EFI
parition.

Change-Id: Ie9f82f3954cc86dc44756256908cc98a2d2d4b11
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>