libpayload: Add ability to unregister output driver

libpayload: Add ability to unregister output driver

This patch adds a console_kill_output_driver() function, which can
remove a previously registered output driver. This is mostly useful when
you overlay some output channel over another, such as when the GDB stub
takes direct control of the UART (and thus has to get rid of the
existing serial output driver).

BUG=chrome-os-partner:18390
TEST=None

Original-Change-Id: I6fce95c22fd15cd321ca6b2d6fbc4e3902b1eac3
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/202561
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit 87680a246429d24e99b7b477b743c357f73b752c)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Change-Id: I50001cee4582c962ceedc215d59238867a6ae95a
Reviewed-on: http://review.coreboot.org/8116
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>