fix(mte): improve ENABLE_FEAT_MTE deprecation warning

ENABLE_FEAT_MTE was deprecated in commit c282384dbb45 ("refactor(mte):
remove mte, mte_perm"), but the check in the build system is only
covering the "ENABLE_FEAT_MTE=1" case.
Fix the check to cover every mentioning of ENABLE_FEAT_MTE.

Change-Id: Ia663f50016455f93ecfdac8aeaba34d9059600ea
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/make_helpers/arch_features.mk b/make_helpers/arch_features.mk
index ceff4ba..675779f 100644
--- a/make_helpers/arch_features.mk
+++ b/make_helpers/arch_features.mk
@@ -310,7 +310,7 @@
 ifeq ($(CTX_INCLUDE_MTE_REGS),1)
         $(warning CTX_INCLUDE_MTE_REGS option is deprecated, Check ENABLE_FEAT_MTE2 usage)
 endif
-ifeq ($(ENABLE_FEAT_MTE),1)
+ifneq ($(ENABLE_FEAT_MTE),)
         $(warning ENABLE_FEAT_MTE option is deprecated, Check ENABLE_FEAT_MTE2 usage)
 endif