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/Makefile b/arch/arm/Makefile
index f5b2b39..f713559 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -281,6 +281,12 @@
 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 @@
   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'