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>