Add CommSerial module

This module implements the comm.h interface for using the serial port
on *nix machines.  This will allow me to use the gdbRemote module from
my OS X machine for test validation.
Automatically connect to mbed serial device

The mbed device usually mounts as /dev/tty.usbmodem* on OS X so find
the first device that matches that expression and open it.  This will
make automated test runs easier.