Introduce config option to select which IO port to send debug output on

Introduce config option to select which IO port to send debug output on

Introduce CONFIG_DEBUG_IO_PORT config option which allow user
to choose on which port to send debug on at configure time.

Bochs users are likely to use 0x402 (default) for debugging purposes,
whereas Xen uses 0xe9, as IO port address.

Signed-off-by: Julian Pidancet <julian.pidancet@citrix.com>