message receive routine for UDP link

def recv_msg(self):
        '''message receive routine for UDP link'''
        self.pre_message()
        s = self.recv()
        if len(s) > 0:
            if self.first_byte:
                self.auto_mavlink_version(s)

        m = self.mav.parse_char(s)
        if m is not None:
            self.post_message(m)

        return m