build(drtm): add DRTM support build option

Added DRTM support build option in the makefiles.
This build option will be used by the DRTM implementation
in the subsequent patches.

Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Change-Id: I15366f86b3ebd6ab2ebcb192753015d547cdddee
diff --git a/Makefile b/Makefile
index b42bdc5..05d97b0 100644
--- a/Makefile
+++ b/Makefile
@@ -833,6 +833,10 @@
     endif
 endif
 
+ifeq ($(DRTM_SUPPORT),1)
+    $(info DRTM_SUPPORT is an experimental feature)
+endif
+
 ################################################################################
 # Process platform overrideable behaviour
 ################################################################################
@@ -1008,6 +1012,7 @@
         HW_ASSISTED_COHERENCY \
         INVERTED_MEMMAP \
         MEASURED_BOOT \
+        DRTM_SUPPORT \
         NS_TIMER_SWITCH \
         OVERRIDE_LIBC \
         PL011_GENERIC_UART \
@@ -1144,6 +1149,7 @@
         HW_ASSISTED_COHERENCY \
         LOG_LEVEL \
         MEASURED_BOOT \
+        DRTM_SUPPORT \
         NS_TIMER_SWITCH \
         PL011_GENERIC_UART \
         PLAT_${PLAT} \