build: add support for passing C preprocessor flags for DTS builds

Useful for adding #ifdefs based on build system provided definitions, or
for adding extra include paths

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/image.mk

@@ -184,6 +184,7 @@ endef
 # $(4) extra DTC flags
 define Image/BuildDTB
 	$(TARGET_CROSS)cpp -nostdinc -x assembler-with-cpp \
+		$(DTS_CPPFLAGS) \
 		-I$(DTS_DIR) \
 		-I$(DTS_DIR)/include \
 		-I$(LINUX_DIR)/include/ \