AP_Motors: add get_throttle_out accessor
Copter: add pre_takeoff throttle feedback

pair-programmed with Randy Mackay
Copter: remain landed until throttle output above 25 percent
Copter: integrate pre-takeoff throttle feedback

This feature slightly revs the motors in response to the pilot's input
before takeoff AltHold, Loiter, AutoTune, PosHold and Sport flight modes

pair-programmed with Randy Mackay
Tools: rename a file with a space in it

spaces in filenames play havoc with unix tools
HAL_Linux: fixed BusTest build
Tools: new dts file for SPI on PXF cape
AP_HAL: added LSM9DSO_G SPI device
HAL_Linux: setup for all 6 SPI devices for PXF cape
AP_InertialSensor: use LSM9DSO_AM device
Tools: fixed DTS Makefile
Tools: added dtbo for PXF cape
Tools: remove unused Linux device tree files
AP_Param: added a method for example programs to set parameters in objects
AP_RangeFinder: added an example program
AP_RangeFinder: removed old test code
Copter: bug fix for throttle failsafe

Missing bracket could cause throttle failsafe to be triggered even when
disabled or motors disarmed
Compass: set_motor_compensation takes instance as first parameter

Also get_motor_compensation uses primary compass
Compass: get_primary method made public
Compass_VRBrain: get_primary made public