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>
diff --git a/bl2/bl2.mk b/bl2/bl2.mk
index f6d69eb..9d75286 100644
--- a/bl2/bl2.mk
+++ b/bl2/bl2.mk
@@ -7,7 +7,10 @@
 BL2_SOURCES		+=	bl2/bl2_main.c				\
 				bl2/${ARCH}/bl2_arch_setup.c		\
 				lib/locks/exclusive/${ARCH}/spinlock.S	\
-				plat/common/${ARCH}/platform_up_stack.S
+				plat/common/${ARCH}/platform_up_stack.S	\
+				${MBEDTLS_COMMON_SOURCES}               \
+				${MBEDTLS_CRYPTO_SOURCES}		\
+				${MBEDTLS_X509_SOURCES}
 
 ifeq (${ARCH},aarch64)
 BL2_SOURCES		+=	common/aarch64/early_exceptions.S