Fix a variable expansion in Arm platform makefiles

The top level makefile defines the PLAT variable, not PLATFORM.
This mistake was causing an empty variable expansion and showing an
incomplete error message.

Change-Id: I5da1275c73c61a7c1823643a76300f255841719d
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

plat/arm/board/juno/platform.mk

@@ -94,7 +94,7 @@ endif
 endif
 
 ifneq (${RESET_TO_BL31},0)
-  $(error "Using BL31 as the reset vector is not supported on ${PLATFORM} platform. \
+  $(error "Using BL31 as the reset vector is not supported on ${PLAT} platform. \
   Please set RESET_TO_BL31 to 0.")
 endif
 

plat/arm/css/sgi/sgi-common.mk

@@ -47,7 +47,7 @@ BL31_SOURCES		+=	${CSS_ENT_BASE}/sgi_ras.c
 endif
 
 ifneq (${RESET_TO_BL31},0)
-  $(error "Using BL31 as the reset vector is not supported on ${PLATFORM} platform. \
+  $(error "Using BL31 as the reset vector is not supported on ${PLAT} platform. \
   Please set RESET_TO_BL31 to 0.")
 endif
 

plat/arm/css/sgm/sgm-common.mk

@@ -46,7 +46,7 @@ BL31_SOURCES		+=	$(SGM_CPU_SOURCES)			\
 				${CSS_SGM_BASE}/sgm_plat_config.c
 
 ifneq (${RESET_TO_BL31},0)
-  $(error "Using BL31 as the reset vector is not supported on ${PLATFORM} platform. \
+  $(error "Using BL31 as the reset vector is not supported on ${PLAT} platform. \
   Please set RESET_TO_BL31 to 0.")
 endif