[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>