Build: Improve BL1 code-sharing

Force symbols to be kept if they are needed in BL1_2, but are not used
in Bl1_1

Change-Id: I5af6f3be552f0f97df704051f03a6994c79c76d5
Signed-off-by: Raef Coles <raef.coles@arm.com>
3 files changed