powerpc: Holly board needs dtbImage target

One of the changes in the bootwrapper makefile introduced the dtbImage
targets for boards that need a simple zImage with a DTB embedded in
them (595be948

, "[POWERPC]
bootwrapper: Build multiple cuImages").  When this was done, it broke
booting on the Holly board as the zImage.holly wrapper did not get the
DTB embedded properly.

This changes the target for the Holly board to a dtbImage so that the
wrapper includes the vmlinux, wrapper bits, and DTB.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>