Force ARCH to arm

It is the only value used for now. No other value works, not even
aarch64.

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index ddd63f1..7caab01 100644
--- a/Makefile
+++ b/Makefile
@@ -14,6 +14,8 @@
 # (we include many *.cmd and *.d files).
 unexport MAKEFILE_LIST
 
+include mk/checkconf.mk
+
 .PHONY: all
 all:
 
@@ -35,7 +37,7 @@
 endif
 
 # Make these default for now
-ARCH            ?= arm
+$(call force,ARCH,arm)
 PLATFORM        ?= vexpress
 # Default value for PLATFORM_FLAVOR is set in plat-$(PLATFORM)/conf.mk
 ifeq ($O,)