sandbox: Improve sandbox serial port keyboard interface

Implements the tstc() interface for the serial driver.  Multiplexing
the console between the serial port and a keyboard uses a polling
method of checking if characters are available; this means that the
serial console must be non-blocking when attempting to read
characters.

Signed-off-by: Taylor Hutt <thutt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>