Free text field for serial port configuration

BR2_TARGET_GENERIC_GETTY_PORT has now a string type instead of choice.
This makes port configuration flexible and compact.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>