This should allow single copter's MOT_ parameter descriptions to be more
easily recognised as belonging to SingleCopter when viewed on the wiki's
arducopter-parameters page

This may fix the issue in which the parameter descriptions are appearing
twice on the arducopter-parameters wiki page

Setting the GPS Failsafe to LAND_EVEN_STABILIZE means the copter will
LAND if it loses GPS even if it's in a manual flight mode like
Stabilize.  With this setting it makes sense to check the GPS quality
before arming even if we're in stabilize mode.

When GPS message is late by 100ms or we are glitching, degrade the GPS
vs inertial nav position error to 10% over 2 seconds instead of
immediately setting it to zero.  This avoids jumpy position estimates
when the GPS misses an update

Previously it was outputting throttle-in which is not as useful in
autonomous modes

this will allow for reverse in steering mode, while also fixing a
problem with AHRS yaw when reversing, and a problem with initial
throttle in steering mode

Required to push back MP Beta Firmwares version to AC3.1-rc5

FS_GPS_ENABLE parameter accepts two new options, 2=AltHold,
3=LandEvenFromStabilize.
If set to 3 the GPS failsafe will trigger and LAND even from manual
flight modes like Stabilize and ACRO.  This is useful for users who want
to ensure their copters can never stray outside the circular fence (the
fence only triggers when it knows it is outside the bounds, and it can't
know this if it has no GPS)

Fourth motor taken from yaw channel's radio_out instead of random
location in memory