[POWERPC] bootwrapper: Add CONFIG_DEVICE_TREE

This provides a way to tell the bootwrapper makefile which device tree to
include by default.  The wrapper can still be invoked standalone to wrap
with a different device tree without reconfiguring the kernel, if that is
desired.

The user will only be asked to provide a device tree if the platform
selects CONFIG_WANT_DEVICE_TREE.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>