ui/ncurses: Expose boot_device configuration through UI

Add a configuration setting for the default boot device.

We change the autoboot option to a select, as we have three possible
behaviours for autoboot:

 ( ) Don't autoboot
 ( ) Autoboot from any disk/network device
 ( ) Only autoboot from a specific disk/network device

The latter option will show a list of possible devices to seelct for
autoboot behaviour.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>