Fix vectored thrust code, flying wing != vectored thrust.

Fix vectored thrust code, flying wing != vectored thrust.

vectored thrust is too generic a term.  examples:
1) fixed wing with ailerons and fixed motors on each wing, yaw the plane
by changing the speeds of the motors
2) fixed wing with ailerons and single motor at rear, yaw the plane by
pivoting the motor left or right

Both of the above scenarios are possible using MIXER_CUSTOM_AIRPLANE.

In baseflight vectored thrust was added in:
https://github.com/multiwii/baseflight/commit/e3585df3a3d8d21481df08367a71bfc18dd5ab7b#diff-2c11cf77e56a7a330caeccadfc1d8bcdR448

then broken in:
https://github.com/multiwii/baseflight/commit/ce0300a3444522ea4fe1f46f7d3a340d38129699#diff-2c11cf77e56a7a330caeccadfc1d8bcdL449

see:
https://github.com/multiwii/baseflight/blob/master/src/mixer.c#L153
https://github.com/multiwii/baseflight/blob/master/src/mixer.c#L328-L329
https://github.com/multiwii/baseflight/blob/master/src/mixer.c#L546-L555