mavproxy_link.py: correct behaviour with old bindings

[peterbarker]

some of these MAV_TYPEs are not known in older bindings, e.g. MAV_TYPE_BATTERY. Avoid fatal errors here when we're working with older bindings

MAV> Detected vehicle 1:1 on link 0
Exception in thread main_loop:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/pbarker/.local/lib/python2.7/site-packages/MAVProxy-1.8.62-py2.7.egg/EGG-INFO/scripts/mavproxy.py", line 1129, in main_loop
  File "/home/pbarker/.local/lib/python2.7/site-packages/MAVProxy-1.8.62-py2.7.egg/EGG-INFO/scripts/mavproxy.py", line 819, in process_master
  File "/home/pbarker/.local/lib/python2.7/site-packages/pymavlink-2.4.39-py2.7.egg/pymavlink/dialects/v20/python2/ardupilotmega.py", line 16823, in parse_buffer
    m = self.parse_char(s)
  File "/home/pbarker/.local/lib/python2.7/site-packages/pymavlink-2.4.39-py2.7.egg/pymavlink/dialects/v20/python2/ardupilotmega.py", line 16762, in parse_char
    self.__callbacks(m)
  File "/home/pbarker/.local/lib/python2.7/site-packages/pymavlink-2.4.39-py2.7.egg/pymavlink/dialects/v20/python2/ardupilotmega.py", line 16750, in __callbacks
    self.callback(msg, *self.callback_args, **self.callback_kwargs)
  File "build/bdist.linux-x86_64/egg/MAVProxy/modules/mavproxy_link.py", line 950, in master_callback
    self.master_msg_handling(m, master)
  File "build/bdist.linux-x86_64/egg/MAVProxy/modules/mavproxy_link.py", line 638, in master_msg_handling
    if self.heartbeat_is_from_autopilot(m):
  File "build/bdist.linux-x86_64/egg/MAVProxy/modules/mavproxy_link.py", line 603, in heartbeat_is_from_autopilot
    mavutil.mavlink.MAV_TYPE_BATTERY,
AttributeError: 'module' object has no attribute 'MAV_TYPE_BATTERY'

[tridge]

i think we're better off forming this as a set() when link module starts up, it would be a lot faster