Copter: Y6 new motor factor fix
Copter: initialise waypoint leash length

Without this initialisation the first RTL could be too aggressive as it
tries to stop too suddenly
AC_WPNav: reduce leash length for stopping

We now limit the target stopping point to 1x the xy leash length while
previously it was 2x.  This is justified because this limit is only used
when the copter is travelling at higher speeds but at higher speeds air
drag tends to make the copter stop more quickly naturally.
PX4: added support for the MPU6000 on the v2.4 Pixhawk
PX4: start all sensor types on PX4

this allows for secondary sensors
build: reduced PX4 warnings and avoid rebuilding when possible
PX4: added -Wno-packed

we know that packed data structures are not as efficient
build: fixed a build cmp warning
PX4: change FMUv2 bootloader USB ID to 0x0011
PX4: added automatic IO firmware update
Copter: 3.1 release notes
Copter: update firmware to 3.1