[POWERPC] bootwrapper: Only print MAC addresses when the node is actually present

Some firmwares (such as PlanetCore) only provide a base MAC address, and
expect the kernel to set certain bits to generate the addresses for the
other ports.  As such, MAC addresses are generated that may not correspond
to actual hardware.

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>