mavproxy_wp: add mission_checksum support

[peterbarker]

Loading the same file multiple times (assuming it is a round-robin copy from ArduPilot) won't send the waypoints again:

MANUAL> wp load Tools/autotest/ArduPlane_Tests/MISSION_CHECKSUM/text.txt
MANUAL> Loaded 373 waypoints from Tools/autotest/ArduPlane_Tests/MISSION_CHECKSUM/text.txt
Got COMMAND_ACK: REQUEST_MESSAGE: ACCEPTED
< MISSION_CHECKSUM {mission_type : 0, checksum : 1752152414}
Mission checksum mismatch
Got MISSION_ACK: TYPE_MISSION: ACCEPTED

MANUAL> 
MANUAL> 
MANUAL> 
MANUAL> Loaded 373 waypoint in 7.46s
Sent all 373 waypoints
Got MISSION_ACK: TYPE_MISSION: ACCEPTED
AP: Flight plan received
< MISSION_CHECKSUM {mission_type : 0, checksum : 4264322242}
Flight battery 100 percent

MANUAL> 
MANUAL> 
MANUAL> wp load Tools/autotest/ArduPlane_Tests/MISSION_CHECKSUM/text.txt
MANUAL> Loaded 373 waypoints from Tools/autotest/ArduPlane_Tests/MISSION_CHECKSUM/text.txt
Got COMMAND_ACK: REQUEST_MESSAGE: ACCEPTED
< MISSION_CHECKSUM {mission_type : 0, checksum : 4264322242}
Mission checksum match