console: Fix UART selection prompt

Without this change, removal of default UART_FOR_CONSOLE entries
under mainboard/ Kconfig will remove this option entirely from
created .config file.

Change-Id: I11422ddb8c51abca177f999936c995ae0c91c459
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5626
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>