powerpc/fsl: Fix build of the dtb embedded kernel images

Commit dc37374b ("powerpc/fsl: Move Freescale device tree files
into fsl folder") moved a lot of device tree files into fsl directory,
fixing Makefile for cuImage target only.  Unfortunately there are other
targets which require embedding a device tree into the kernel image
(e.g. dtbImage.%).  So use a more generic approach.
Signed-off-by: Alessio Igor Bogani <alessio.bogani@elettra.eu>
[scottwood: cleaned up commit message]
Signed-off-by: Scott Wood <oss@buserror.net>

arch/powerpc/boot/Makefile

@@ -362,9 +362,6 @@ $(obj)/cuImage.initrd.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 $(obj)/cuImage.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 	$(call if_changed,wrap,cuboot-$*,,$(obj)/$*.dtb)
 
-$(obj)/cuImage.%: vmlinux $(obj)/fsl/%.dtb $(wrapperbits)
-	$(call if_changed,wrap,cuboot-$*,,$(obj)/fsl/$*.dtb)
-
 $(obj)/simpleImage.initrd.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 	$(call if_changed,wrap,simpleboot-$*,,$(obj)/$*.dtb,$(obj)/ramdisk.image.gz)
 
@@ -381,6 +378,9 @@ $(obj)/treeImage.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 $(obj)/%.dtb: $(src)/dts/%.dts FORCE
 	$(call if_changed_dep,dtc)
 
+$(obj)/%.dtb: $(src)/dts/fsl/%.dts FORCE
+	$(call if_changed_dep,dtc)
+
 # If there isn't a platform selected then just strip the vmlinux.
 ifeq (,$(image-y))
 image-y := vmlinux.strip