arm/dt: Add dtb make rule

Add a make rule to compile dt blobs for ARM.
Signed-off-by: NRob Herring <rob.herring@calxeda.com>
Acked-by: NShawn Guo <shawn.guo@freescale.com>
Tested-by: NJason Liu <jason.hui@linaro.org>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

arch/arm/Makefile

@@ -281,6 +281,12 @@ zImage Image xipImage bootpImage uImage: vmlinux
 zinstall uinstall install: vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $@
 
+%.dtb:
+	$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
+
+dtbs:
+	$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
+
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
@@ -297,6 +303,7 @@ define archhelp
   echo  '  uImage        - U-Boot wrapped zImage'
   echo  '  bootpImage    - Combined zImage and initial RAM disk' 
   echo  '                  (supply initrd image via make variable INITRD=<path>)'
+  echo  '  dtbs          - Build device tree blobs for enabled boards'
   echo  '  install       - Install uncompressed kernel'
   echo  '  zinstall      - Install compressed kernel'
   echo  '  uinstall      - Install U-Boot wrapped compressed kernel'

arch/arm/boot/Makefile

@@ -59,6 +59,12 @@ $(obj)/zImage:	$(obj)/compressed/vmlinux FORCE
 
 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) \