[POWERPC] Use embedded libfdt in the bootwrapper

This incorporates libfdt (from the source embedded in an earlier
commit) into the wrapper.a library used by the bootwrapper.  This
includes adding a libfdt_env.h file, which the libfdt sources need in
order to integrate into the bootwrapper environment, and a
libfdt-wrapper.c which provides glue to connect the bootwrapper's
abstract device tree callbacks to the libfdt functions.

In addition, this changes the various wrapper and platform files to
use libfdt functions instead of the older flatdevtree.c library.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>