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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 read part.

It stores the current blocking properties before disabling blocking and
restores them after reading. The timeout is implemented as previously
by retrying every millisecond until the timeout is reached or enough
characters are available.

Corresponding to flashrom svn r1661.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>