[POWERPC] bootwrapper: Add get_path()

This will be used by the PlanetCore firmware support to construct
a linux,stdout-path from the serial node that it finds.

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>