kernel-fitimage: Adjust order of dtb/dtbo files

The dtb files must be before the dtbo files, otherwise the overlays may
not be applied correctly.

From Bruce Ashfield:

  We can split between dtbs and dtbos, they just need to be sorted
  for reproducibility reasons.

  Of course, this was only working by luck previously (before the
  sort), since it has always been gathering dtbs and dtbo's with
  find, depending on filesystem ordering for the order in the
  fitimage).

Signed-off-by: Sandeep Gundlupet Raju <sandeep.gundlupet-raju@amd.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>