image.mk: add a BuildDTB define

Add a Image/BuildDTB command that takes the path to an dts file and
applies the CPP using the default include directories, then uses dtc
to compile the dts as the specified output file.
Additional CPP and DTC flags can be supplied as additional arguments.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46389 3c298f89-4303-0410-b956-a3cf2f4a3e73

include/image.mk

@@ -126,6 +126,22 @@ define Image/BuildKernel/MkFIT
 	PATH=$(LINUX_DIR)/scripts/dtc:$(PATH) mkimage -f $(KDIR)/fit-$(1).its $(KDIR)/fit-$(1)$(7).itb
 endef
 
+# $(1) source dts file
+# $(2) target dtb file
+# $(3) extra CPP flags
+# $(4) extra DTC flags
+define Image/BuildDTB
+	$(CPP) -nostdinc -x assembler-with-cpp \
+		-I$(LINUX_DIR)/arch/$(ARCH)/boot/dts \
+		-I$(LINUX_DIR)/arch/$(ARCH)/boot/dts/include \
+		-undef -D__DTS__ $(3) \
+		-o $(2).tmp $(1)
+	$(LINUX_DIR)/scripts/dtc/dtc -O dtb \
+		-i$(dir $(1)) $(4) \
+		-o $(2) $(2).tmp
+	$(RM) $(2).tmp
+endef
+
 define Image/mkfs/jffs2/sub
 		$(STAGING_DIR_HOST)/bin/mkfs.jffs2 $(3) --pad -e $(patsubst %k,%KiB,$(1)) -o $(KDIR)/root.jffs2-$(2) -d $(TARGET_DIR) -v 2>&1 1>/dev/null | awk '/^.+$$$$/'
 		$(call add_jffs2_mark,$(KDIR)/root.jffs2-$(2))