Add example implementation of a trivial blocking read function as a
part of Barebox's serdev API.

NOTE: This code has not been tested against real devices, so use it at
your own risk.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>