Allow -Werror to be temporarily disabled using "make NO_WERROR=1"

src/Makefile

 #
 CFLAGS		+= -I include -I arch/$(ARCH)/include -I . -DARCH=$(ARCH)
 CFLAGS		+= -Os -ffreestanding
-CFLAGS		+= -Wall -W -Werror
+CFLAGS		+= -Wall -W
 CFLAGS		+= -g
 CFLAGS		+= $(EXTRA_CFLAGS)
 ASFLAGS		+= $(EXTRA_ASFLAGS)
 LDFLAGS		+= $(EXTRA_LDFLAGS)
 
+ifneq ($(NO_WERROR),1)
+CFLAGS		+= -Werror
+endif
+
 # CFLAGS for specific object types
 #
 CFLAGS_c	+=