image.mk: use cross toolchain cpp for processing dts file instead of relying on the host compiler

Signed-off-by: Felix Fietkau <[email protected]>

SVN-Revision: 47937

include/image.mk

@@ -142,7 +142,7 @@ endef
 # $(3) extra CPP flags
 # $(4) extra DTC flags
 define Image/BuildDTB
-	$(CPP) -nostdinc -x assembler-with-cpp \
+	$(TARGET_CROSS)cpp -nostdinc -x assembler-with-cpp \
 		-I$(DTS_DIR) \
 		-I$(DTS_DIR)/include \
 		-undef -D__DTS__ $(3) \