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/bl1/bl1.mk b/bl1/bl1.mk
index a026499..41ee1a7 100644
--- a/bl1/bl1.mk
+++ b/bl1/bl1.mk
@@ -13,7 +13,10 @@
 				lib/cpus/errata_report.c		\
 				lib/el3_runtime/${ARCH}/context_mgmt.c	\
 				plat/common/plat_bl1_common.c		\
-				plat/common/${ARCH}/platform_up_stack.S
+				plat/common/${ARCH}/platform_up_stack.S \
+				${MBEDTLS_COMMON_SOURCES}		\
+				${MBEDTLS_CRYPTO_SOURCES}		\
+				${MBEDTLS_X509_SOURCES}
 
 ifeq (${ARCH},aarch64)
 BL1_SOURCES		+=	lib/el3_runtime/aarch64/context.S