TSP: Make the platform-specific makefile mandatory

The Test Secure-EL1 Payload implementation should always have a
platform-specific component.  Therefore, there should always
be a platform-specific sub-makefile for the TSP.  If there is
none then assume TSP is not supported on this specific platform
and throw an error at build time if the user tries to compile it.

Change-Id: Ibfbe6e4861cc7786a29f2fc0341035b852925193

bl32/tsp/tsp.mk

@@ -51,5 +51,12 @@ vpath %.c ${BL32_ROOT}
 vpath %.c ${BL32_ROOT}/${ARCH}
 vpath %.S ${BL32_ROOT}/${ARCH}
 
-# Include an optional platform-specific TSP Makefile
--include bl32/tsp/tsp-${PLAT}.mk
+# Include the platform-specific TSP Makefile
+# If no platform-specific TSP Makefile exists, it means TSP is not supported
+# on this platform.
+TSP_PLAT_MAKEFILE := bl32/tsp/tsp-${PLAT}.mk
+ifeq (,$(wildcard ${TSP_PLAT_MAKEFILE}))
+  $(error TSP is not supported on platform ${PLAT})
+else
+  include ${TSP_PLAT_MAKEFILE}
+endif