Copter: move relay from ch6 to ch7/ch8
Copter: correct EKF_CHECK parameter description
BaroGlitch: increase max accel to 15m/s/s
Copter: rename EKF check's error message to dataflash

It checks more than the compass so the old error message was misleading.
GCS_MAVLink: added battery2 message
GCS_MAVLink: rebuild MAVLink headers
AP_BattMonitor: added 2nd battery voltage monitoring
GCS_MAVLink: added send_battery2()
Plane: added 2nd battery monitoring support
Plane: move battery init to after parameter load
HAL_PX4: don't report power status change if in first 5 seconds
RC_Channel: fixed k_manual for set_servo_limit()

this allows for failsafe takeover of k_manual channels, which fixes
the behaviour for ignition failsafe
autotest: added -N option to sim_vehicle.sh

useful for unusual builds
AP_InertialSensor: improved HIL timing consistancy

try to keep the average time for each sample right
SITL: make it possible to use SITL-HIL

strangely enough, using HIL in SITL is a great way to debug HIL!
Plane: zero loc structure in HIL_STATE handling

this avoids av AVR bug in HIL
AP_GPS: make setHIL take const references
Plane: prepare for 3.1.0-beta2
AP_Airspeed: added HIL support
Plane: added airspeed simulation in HIL