serial: add an uart-bridge driver

Add a reusable uart-bridge driver that echoes data between two serial
devices. It's mainly meant to be used with one of the devices being an
USB CDC-ACM, the data is buffered, there's a pause feature to stop the
USB endpoint when the hardware UART is filling up to avoid overrun and
there's a helper function used to propagate the USB uart configuration
to the hardware one.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>