refactor(build): avoid using values for comparison

With changes to refactor to use first platform makefiles
then parse arch_features.mk file 'ENABLE_RME' will be initialised
only when we define during build or at arch_features.mk thus
making comparison of 'ENABLE_RME' to '0' incorrect.

So keep BRBE disabled when RME is enabled at main makefile level.

Change-Id: I7e3d99eb444678d63585bd5971ada627cfc4fcc9
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/Makefile b/Makefile
index 417e512..7f781a0 100644
--- a/Makefile
+++ b/Makefile
@@ -623,6 +623,9 @@
 ################################################################################
 # FEAT_RME
 ifeq (${ENABLE_RME},1)
+	# RME doesn't support BRBE
+	ENABLE_BRBE_FOR_NS := 0
+
 	# RME doesn't support PIE
 	ifneq (${ENABLE_PIE},0)
                 $(error ENABLE_RME does not support PIE)