[POWERPC] add target for building .dtb files

Call dtc from the Makefile instead of the wrapper script so that the dt
blobs can be generated with a simple make invocation.

Using this patch allows board ports to trigger automatic building of .dtb
files by adding them to the image-y target list.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index ab5cfe8..dd80825 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -164,7 +164,7 @@
 $(BOOT_TARGETS): vmlinux
 	$(Q)$(MAKE) ARCH=ppc64 $(build)=$(boot) $(patsubst %,$(boot)/%,$@)
 
-bootwrapper_install:
+bootwrapper_install %.dtb:
 	$(Q)$(MAKE) ARCH=ppc64 $(build)=$(boot) $(patsubst %,$(boot)/%,$@)
 
 define archhelp