dts/Makefile: pass -undef -D__DTS__ to cpp

This brings U-Boot's cpp invocation into line with the way the Linux
kernel invokes cpp on device trees. Consistency will be useful to ensure
*.dts is portable between the two.

-undef also has the added advantage of not defining "linux", so DT
property names such as "linux,keymap" don't get mangled.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

dts/Makefile

@@ -21,7 +21,7 @@ DTS_INCDIRS =  $(SRCTREE)/board/$(VENDOR)/$(BOARD)/dts
 DTS_INCDIRS += $(SRCTREE)/board/$(VENDOR)/dts
 DTS_INCDIRS += $(SRCTREE)/arch/$(ARCH)/dts
 
-DTS_CPPFLAGS := -x assembler-with-cpp \
+DTS_CPPFLAGS := -x assembler-with-cpp -undef -D__DTS__ \
 		-nostdinc $(addprefix -I,$(DTS_INCDIRS))
 
 DTC_FLAGS := -R 4 -p 0x1000 \