Plane: changed preflight calibration to prevent accel cal from MP

MissionPlanner sends param1=1 and param3=1 for preflight
calibration. This was having the effect of redoing the accel
calibration as 1D cal on every flight!
Rover: make preflight calibration follow plane conventions
Plane: prepare for 3.1.2beta3
Plane: if SKIP_GYRO_CAL is set the do a gyro cal on 3D accel cal

otherwise the user has no opportunity to do a gyro calibration
Copter: only allow one preflight calibration at a time
AP_Math: add yaw 293, pitch 68, roll 180 rotation
AP_Math: add new rotation to example rotation sketch
Compass: add new rotation to ORIENT param description
AP_InertialSensor: allow gyro calibration to take up to 30 seconds

this gives time for the user time to get their battery hatches on and
hold the model steady. 5 seconds was just far too short
AP_NavEKF: add another health check in the EKF

if SV, SP and SH are all off then the most likely cause is divergence
of the EKF. This was done based on a flight log with bad gyro cal
Replay: cope with new airspeed log msg
AP_NavEKF: added getStaticMode() function
DataFlash: added staticmode to EKF4 message

useful when diagnosing logs
AP_Math: fixed build warning
RC_Channel: added set_radio_trimmed()

take into account trim value in set_radio()
Plane: use set_radio_trimmed() for flaperons

See discussion in issue #1425
GCS_Mavlink: New message def: MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: include units in CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: code generation from XML for MAV_CMD_CONTINUE_AND_CHANGE_ALT.
AP_Mission: support for MAV_CMD_CONTINUE_AND_CHANGE_ALT