DataFlash: don't try to create a directory that exists

this is an attempt to avoid microSD card corruption. The most common
corruption is two 'logs' directories, which may indicate an issue with
trying to create it a 2nd time.
AP_InertialSensor: another attempt at fixing the spurious bad gyro health warnings

cope with _get_sample() not being called for a while
Copter: 3.1 release notes
Copter: remove unnecessary MPU6K filter definition

This should also help kick off the AC3.1 rebuild
TradHeli: Fix for yaw offset switching.  Yaw Offset should switch on and off with the main motor, not waiting until the rotor has reached full speed.
RC_Channel: improved docs for RCn_FUNCTION=0
Rover: added timestamps to logging and new STER logging

much more accurate logging for better analysis
Test: testing commit

This is a test commit
Copter: removed some unnecessary fields in GCS.h

save some memory
Rover: use common GCS.h
Copter: moved GCS.h to common library
Plane: use common GCS.h
GCS_MAVLink: import new mavlink XML
GCS_MAVLink: re-generate MAVLink headers
GCS_MAVLink: started adding MAVLink log download
DataFlash: added functions for load download support
Plane: moved ap_message into GCS.h
Copter: moved ap_message into GCS.h
Rover: moved ap_message into GCS.h
GCS_MAVLink: new headers for log download