AP_Scheduler: improved the accuracy of the scheduling code

prevent accumulated micros() errors in run()
autotest: speed up build_all_px4.sh

only build archives once
AP_Scheduler: show slips only for SCHED_DEBUG>1

this allows us to see PERF reports in copter without console writes
slowing things down
HAL_AVR: reduce the latency of semaphore waits

this reduces the average cost of waiting for the MPU6000 semaphore
from the main loop
Rover: prepare for 2.43 release
autotest: fixed ACRO attitude locking in plane
AP_GPS: changed fake GPS fix to have a latitude/longitude

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
HAL_PX4: disable checks for in_timerprocess

these are not valid in this form for the way the timer thread works on
PX4.
Plane: release 2.75beta3
Rover: remove some unused functions
MAVLink: added MAV_CMD_DO_SET_CAM_TRIGG_DIST
GCS_MAVLink: regenerate mavlink headers
AP_Camera: added set_trigger_distance() call
Plane: support MAV_CMD_DO_SET_CAM_TRIGG_DIST
Copter: support MAV_CMD_DO_SET_CAM_TRIGG_DIST
Rover: support MAV_CMD_DO_SET_CAM_TRIGG_DIST
Iris: Update flight modes
Iris: Set FS_BATT_VOLTAGE
AP_SpdHgtControl: added FightStage parameter to update_pitch_throttle()

will be used for special landing handling
AP_TECS: added initial flight_stage handling