refactor(measured-boot): cleanup Event Log makefile

The Event Log sources are added to the source-list of BL1 and BL2
images in the Event Log Makefile. It doesn't seem correct since
some platforms only compile Event Log sources for BL2.
Hence, moved compilation decision of Event Log sources to the
platform makefile.

Change-Id: I1cb96e24d6bea5e091d08167f3d1470d22b461cc
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>