console: Add console for GDB

Connection of UARTs to GDB stub got lost in the console transition
process, bring it back. In theory, GDB stub should work also over
usbdebug, but that solution is not really tested at all yet.

Change-Id: I90e05e8132889e788b92e055ee191f35add43bbc
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5343
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>