sgi/build-test-secureboot: align to the changes in busybox boot test script

The busybox boot test script has reorganized the disk image preparation
code. Do the same for secure busybox boot as well. In addition to this,
add the build override macros for UEFI (SECURE_STORAGE_ENABLE=TRUE and
SECURE_BOOT_ENABLE=TRUE") and do minor cleanup as well.

Change-Id: I5dd709d96d20bdf8cf06d048fb8d0467ba15ae2b
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>