installer: add an XO installer (written in forth)

This uses OFW's menu support to display an installation
method.  Currently doesn't do anything interesting, other
than display available install target devices on a system.

Stick this on a usb stick (as /boot/olpc.fth), boot up your XO,
and run 'instmenu' from the OFW prompt.

Signed-off-by: Andres Salomon <dilinger@queued.net>