Introduce serialport_config().

This allows to easily reconfigure a serial port as needed in
the Bus Pirate speedup patch.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1717 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1