build: Pull -DBUILD_TAG into separate ifdef

Currently the base setting for CFLAGS is split in two possibilities,
one with -DBUILD_TAG appended at the end and one without, the rest of
CFLAGS is the same in both cases. Change this so CFLAGS are always set
and the -DBUILD_TAG is appended in separate ifdef.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>

config.mk

@@ -250,11 +250,10 @@ CPPFLAGS += -I$(TOPDIR)/include
 CPPFLAGS += -fno-builtin -ffreestanding -nostdinc	\
 	-isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS)
 
-ifdef BUILD_TAG
-CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes \
-	-DBUILD_TAG='"$(BUILD_TAG)"'
-else
 CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes
+
+ifdef BUILD_TAG
+CFLAGS += -DBUILD_TAG='"$(BUILD_TAG)"'
 endif
 
 CFLAGS_SSP := $(call cc-option,-fno-stack-protector)