of/powerpc: Use generic rule to build dtb's

Modify arch/powerpc/boot/Makefile to use dtc command in
scripts/Makefile.lib
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/powerpc/boot/Makefile

@@ -35,7 +35,7 @@ endif
 
 BOOTCFLAGS	+= -I$(obj) -I$(srctree)/$(obj)
 
-DTS_FLAGS	?= -p 1024
+DTC_FLAGS	?= -p 1024
 
 $(obj)/4xx.o: BOOTCFLAGS += -mcpu=405
 $(obj)/ebony.o: BOOTCFLAGS += -mcpu=405
@@ -332,10 +332,8 @@ $(obj)/treeImage.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 	$(call if_changed,wrap,treeboot-$*,,$(obj)/$*.dtb)
 
 # Rule to build device tree blobs
-DTC = $(objtree)/scripts/dtc/dtc
-
-$(obj)/%.dtb: $(dtstree)/%.dts
-	$(DTC) -O dtb -o $(obj)/$*.dtb -b 0 $(DTS_FLAGS) $(dtstree)/$*.dts
+$(obj)/%.dtb: $(src)/dts/%.dts
+	$(call cmd,dtc)
 
 # If there isn't a platform selected then just strip the vmlinux.
 ifeq (,$(image-y))