Move OS-dependent serial code from buspirate_spi.c to serial.c

While at it, also rename a few functions to make it obvious
that they are generic and not specific to the Bus Pirate.

Corresponding to flashrom svn r830.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Patrick Georgi <patrick.georgi@coresystems.de>