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It seems useful to have a generic and platform-independent method to
read and write to a serial port without blocking. This is the write part.

This allows to get rid of the explicit temporary disabling of blocking I/O in
serprog's sp_synchronize().

Corresponding to flashrom svn r1662.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>