fix(build): clean el3 test payload image

The clean command also cleans the optional el3 test payload image even
if the platform does not support the el3_payload test. This will result
in makefile  error, since the el3_payload build system will try to
include the platform.mk file for the platform - which will be missing in
case of a unsupported platform. Hence, add a condition in the clean and
distclean command.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I27e1f3f66042b90ab1b0ee4780c587c6fd2a1f00
diff --git a/Makefile b/Makefile
index ff8ac27..55f9690 100644
--- a/Makefile
+++ b/Makefile
@@ -320,14 +320,18 @@
 clean:
 			@echo "  CLEAN"
 			${Q}rm -rf ${BUILD_PLAT}
+ifneq ($(wildcard ${EL3_PAYLOAD_PLAT_MAKEFILE_FULL}),)
 			${MAKE} -C el3_payload clean
+endif
 
 .PHONY: realclean distclean
 realclean distclean:
 			@echo "  REALCLEAN"
 			${Q}rm -rf ${BUILD_BASE}
 			${Q}rm -f ${CURDIR}/cscope.*
+ifneq ($(wildcard ${EL3_PAYLOAD_PLAT_MAKEFILE_FULL}),)
 			${MAKE} -C el3_payload distclean
+endif
 
 .PHONY: checkcodebase
 checkcodebase:		locate-checkpatch