build(rcar): separate BL2 and BL31 SREC generation

This small change creates individual Make non-phony targets for the Bl2
and BL31 SREC binaries to avoid rebuilding them unnecessarily.

Change-Id: Ibb8880bb5c00a0956fc78d252fcc56391fbfe439
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed