GCS_MAVLink: merged with upstream
GCS_MAVLink: regenerate headers
AP_AHRS: ensure get_position() fills in flags
AP_NavEKF: ensure get_position() fills in flags
AP_Terrain: added current_height to TERRAIN_REPORT

and send TERRAIN_REPORT even when downloading TERRAIN_DATA
Plane: improved TERRAIN_FOLLOW docs
GCS_MAVLink: added missing generated MAVLink headers
Copter: fixed auto-doc prefix for TERRAIN
Plane: fixed auto-doc prefix for TERRAIN
AP_Terrain: expand parameter docs
AP_Terrain: prevent valgrind error when terrain not available
Notify: APM2 arming buzzer to long tone

Arming tone is a solid 3 second tone so it is consistent with Pixhawk
arming tone
Copter: fix arming tone to sound before arming

Also fixed bug in which vehicle could appear to be armed (according to
LEDs) but is not.  This could occur if pilot had raised the throttle
very high during the calibration that happens just before arming.