GitLab

Move all TBBR-specific stuff out of the tool's makefile into a
sub-makefile. This will make it easier to define and select an alternate
chain of trust in the future.

Change-Id: I92e366a1999b74cf51127d1771b64b807cd94b29
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>