Update USB VCP example.
# port = '/dev/ttyACM0'
# sp = serial.Serial(port, baudrate=115200, bytesize=serial.EIGHTBITS, parity=serial.PARITY_NONE,
# xonxoff=False, rtscts=False, stopbits=serial.STOPBITS_ONE, timeout=None, dsrdtr=True)
+# sp.setDTR(True) # dsrdtr is ignored on Windows.
# sp.write("snap")
# sp.flush()
# size = struct.unpack('<L', sp.read(4))[0]
