refactor(build): allow mandatory feats disabling

Currently any arch FEAT_* can be enabled from:
 - command line build options
 - platform makefile
 - from arch_features.mk

These are in order. However, mandatory features are enforced from
arch_features.mk and platform makefile can't override them.

Allow command line options or platforms makefile to disable any
mandatory features.

Change-Id: I6fdca1a3d0b405a88cd7a20309e0c1eecd57a650
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>

make_helpers/arch_features.mk

@@ -20,48 +20,48 @@
 
 # Enable the features which are mandatory from ARCH version 8.1 and upwards.
 ifeq "8.1" "$(word 1, $(sort 8.1 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_PAN				:=	1
-ENABLE_FEAT_VHE				:=	1
+ENABLE_FEAT_PAN				?=	1
+ENABLE_FEAT_VHE				?=	1
 endif
 
 # Enable the features which are mandatory from ARCH version 8.2 and upwards.
 ifeq "8.2" "$(word 1, $(sort 8.2 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_RAS				:=	1
+ENABLE_FEAT_RAS				?=	1
 endif
 
 # Enable the features which are mandatory from ARCH version 8.4 and upwards.
 ifeq "8.4" "$(word 1, $(sort 8.4 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_SEL2			:=	1
-ENABLE_TRF_FOR_NS			:=	1
-ENABLE_FEAT_DIT				:=	1
+ENABLE_FEAT_SEL2			?=	1
+ENABLE_TRF_FOR_NS			?=	1
+ENABLE_FEAT_DIT				?=	1
 endif
 
 # Enable the features which are mandatory from ARCH version 8.5 and upwards.
 ifeq "8.5" "$(word 1, $(sort 8.5 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_RNG				:=	1
-ENABLE_FEAT_SB				:=	1
+ENABLE_FEAT_RNG				?=	1
+ENABLE_FEAT_SB				?=	1
 
 # Enable Memory tagging, Branch Target Identification for aarch64 only.
 ifeq ($(ARCH), aarch64)
-	mem_tag_arch_support		:= 	yes
+	mem_tag_arch_support		?= 	yes
 endif #(ARCH=aarch64)
 
 endif
 
 # Enable the features which are mandatory from ARCH version 8.6 and upwards.
 ifeq "8.6" "$(word 1, $(sort 8.6 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_ECV				:=	1
-ENABLE_FEAT_FGT				:=	1
+ENABLE_FEAT_ECV				?=	1
+ENABLE_FEAT_FGT				?=	1
 endif
 
 # Enable the features which are mandatory from ARCH version 8.7 and upwards.
 ifeq "8.7" "$(word 1, $(sort 8.7 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_HCX				:=	1
+ENABLE_FEAT_HCX				?=	1
 endif
 
 # Enable the features which are mandatory from ARCH version 8.9 and upwards.
 ifeq "8.9" "$(word 1, $(sort 8.9 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
-ENABLE_FEAT_TCR2			:=	1
+ENABLE_FEAT_TCR2			?=	1
 endif
 
 #