[flashrom] [PATCH]various serial fixes

[Patrick Georgi]

Hi,

attached patch changes:
- replace the hand written serial input flush routine with the termios one.
- serialport_discard_read isn't necessary anymore - it just wrapped
sp_flush_incoming with no extra value.
- serialport_read and serialport_write would misbehave if read or write
didn't process everything in one go.

All untested as I don't have a system where these are used. Just wanted
these issues out in public.

Signed-off-by: Patrick Georgi