arm/dt: Add dtb make rule

Add a make rule to compile dt blobs for ARM.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Tested-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/arch/arm/boot/Makefile b/arch/arm/boot/Makefile
index 9128fdd..a1edfd5 100644
--- a/arch/arm/boot/Makefile
+++ b/arch/arm/boot/Makefile
@@ -59,6 +59,12 @@
 
 endif
 
+# Rule to build device tree blobs
+$(obj)/%.dtb: $(src)/dts/%.dts
+	$(call cmd,dtc)
+
+$(obj)/dtbs: $(addprefix $(obj)/, $(dtb-y))
+
 quiet_cmd_uimage = UIMAGE  $@
       cmd_uimage = $(CONFIG_SHELL) $(MKIMAGE) -A arm -O linux -T kernel \
 		   -C none -a $(LOADADDR) -e $(STARTADDR) \