[POWERPC] bootwrapper: Add PlanetCore firmware support

This is a library that board code can use to extract information from the
PlanetCore configuration keys.  PlanetCore is used on various boards from
Embedded Planet.

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>