juno: Make the build fail when RESET_TO_BL31 != 0

Enabling BL3-1 entrypoint as the CPU reset vector in place of the
BL1 entrypoint is not supported on Juno at the moment. This patch
modifies the build system so that the build will crash straight
away if the user attempts to use this feature on Juno.

Change-Id: I641275fa37c2892fae7b944219b406f945e7e1b4

plat/juno/platform.mk

@@ -68,3 +68,8 @@ BL31_SOURCES		+=	drivers/arm/cci400/cci400.c		\
 				plat/juno/plat_gic.c			\
 				plat/juno/scpi.c			\
 				plat/juno/smc_arm.c
+
+ifneq (${RESET_TO_BL31},0)
+  $(error "Using BL3-1 as the reset vector is not supported on Juno. \
+  Please set RESET_TO_BL31 to 0.")
+endif