Make -march settings part of USERCFLAGS

The current -march settings will cause an error if the user a
conflicting flags such as -mcpu=cortex-a15. Use USERCFLAGS instead
of CFLAGS so that a default is provided but can be overridden.

Change-Id: I8ffecd6bfd7ad5794486a5d14d27bce3170898d3
Acked-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -63,11 +63,11 @@ ifeq ($(shell echo $(ARCH) | sed -e 's/arm.*/arm/'),arm)
 	DEFINES      := -DCONFIG_ARMV$(ARMV)
 
 	ifeq ($(ARMV),6)
-		CFLAGS += -march=armv6
+		USERCFLAGS += -march=armv6
 	endif
 
 	ifeq ($(ARMV),7)
-		CFLAGS += -march=armv7-a
+		USERCFLAGS += -march=armv7-a
 	endif
 endif