show available ports

def cmd_link_ports(self):
        '''show available ports'''
        ports = mavutil.auto_detect_serial(preferred_list=['*FTDI*',"*Arduino_Mega_2560*", "*3D_Robotics*", "*USB_to_UART*", '*PX4*', '*FMU*'])
        for p in ports:
            print("%s : %s : %s" % (p.device, p.description, p.hwid))