bl2-el3: Don't compile BL1 when BL2_AT_EL3 is defined in FVP

bl2-el3: Don't compile BL1 when BL2_AT_EL3 is defined in FVP

This patch modifies the makefiles to avoid the definition
of BL1_SOURCES and BL2_SOURCES in the tbbr makefiles, and
it lets to the platform makefiles to define them if they
actually need these images. In the case of BL2_AT_EL3
BL1 will not be needed usually because the Boot ROM will
jump directly to BL2.

Change-Id: Ib6845a260633a22a646088629bcd7387fe35dcf9
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>