start/stop RC calibration

def cmd_rccal(self, args):
        '''start/stop RC calibration'''
        if len(args) < 1:
            self.print_cal_usage()
            return

        if (args[0] == "start"):
            if len(args) > 1:
                self.num_channels = int(args[1])
            print("Calibrating %u channels" % self.num_channels)
            print("WARNING: remove propellers from electric planes!!")
            print("Push return when ready to calibrate.")
            raw_input()

            self.clear_rc_cal()
            self.calibrating = True
        elif (args[0] == "done"):
            self.calibrating = False
            self.apply_rc_cal()
        else:
            self.print_cal_usage()