AP_Vehicle: added stall_prevention variable
AP_TECS: use aparm.stall_prevention
Plane: added STALL_PREVENTION option

this will allow users to disable it if it proves to be a problem with
no airspeed sensor
Plane: small improvement to takeoff docs
AP_L1_Control: update NAV_L1_PERIOD docs
autotest: ensure we get the BIN flight logs even on mission failure
autotest: save core and elf files

try to catch floating pt exception causes
SITL: abort on floating point error

this creates a core file
build: put elf file in current directory

makes it more obvious where the build ends up
AP_Airspeed: expose raw pressure for logging
Plane: log raw pressure and offset
AP_Airspeed: added ARSPD_SKIP_CAL parameter

allows airspeed calibration to be skipped for easier startup
Plane: changed for new AP_Airspeed API
autotest: fixed buildlogs location for *.BIN
APM_Control: changed default I gain for roll/pitch controllers to non-zero

too many people are still not tuning. At least this will give them a
chance to get their aircraft back in a cross-wind
Added name to GIT_Success.txt
Copter: Updated GCS_FAILSAFE Parameter Description
AP_Compass: fixed HMC5883's initialisation code
AP_HAL: added Zynq platform type
HAL_Linux: Basic Zynq Linux platform support

Preliminary support for Zynq/Linux on the 'ZyboPilot' HW platform.

see https://github.com/trjw/ZyboPilot-bsp for the PetaLinux / Vivado project
files.

At this stage CPPM/Pulse in and PWM out work, and in CLI mode the passthru
test works correctly.

The platform has passed minimal smoke testing in HIL mode.

ZYNQ IS NOT FLIGHT TESTED YET!  FLY THIS PLATFORM AT YOUR OWN RISK!

Signed-off-by: John Williams <john@whelanwilliams.net>