This module creates produces an output file named osXX.usb. This file can
be used with the command "olpc-update --usb" to update the software on a
laptop.

This module requires the versioned_fs option in the base module to be enabled.
(the update system revolves around the fact that a versioned fs is used)

The .usb file is actually an ISO image in disguise. In the future, we should
enable squashfs in the XO kernel and distribute a squashfs image instead.