Copter: allow starting autotune from flightmode switch

ch5 switch to AutoTune is equivalent to starting autotune via sw7/8.
Copter: minor reformat of control_autotune
build_devrelease: build all frames for jons release
AP_Mount: Added lead filter
DataFlash: add UBX3 message for accuracy logging
AP_GPS: Add uBlox accuracy metrics interface and logging
AP_NavEKF: Add parameter that specifies EKF-to-DCM/INAV fallback strictness
AP_Arming: remove abbreviations from GCS messages
Copter: ensure lat, lon, alt all zero is handled

Use default position when waypoint lat/lon are zero,
and default altitude when waypoint alt is zero, for
both spline and straight-line waypoints.

Fixes #1145.
AutoTest: copter test that zero alt defaults to current alt
AC_WPNav: minor fix to comments
Copter: clarify pv_location_to_vector param name

Also update function description
Plane: add support for generating a PX4 mixer for failsafe

this creates APM/MIXER.MIX which will be used if the FMU dies to
provide manual control over RC
Plane: added OVERRIDE_CHAN parameter

this allows both for testing PX4IO override on the ground, and for
forced manual override while flying, including re-arming after in-air
reboot
RC_Channel: added channel_function()

this is used in the plane mixing code
HAL_PX4: if there are no enabled channels don't send to PX4IO

this makes it possible to disable PWM output to IO to test override
Plane: improved FLAPIN_CHANNEL docs
Plane: make OVERRIDE_CHAN behaviour exactly match FLTMODE6

this provides a convenient "real manual" flight mode
Plane: handle trims and reversals in px4 mixer creation
Plane: limit RC config to 8 channels

this is a limitation of current px4io.c