[POWERPC] bootwrapper: flatdevtree fixes

1. ft_create_node was returning the internal pointer rather than a phandle.
2. ft_find_device_rel was treating a "top" phandle of NULL as an error,
rather than as the root of the tree.  The old, absolute ft_find_device
is removed, and the relative version is renamed to ft_find_device().

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>