set motors via an commander setpoint

[knmcguire]

Currently people can change the motor speed by a parameter: https://github.com/bitcraze/crazyflie-firmware/blob/master/src/drivers/src/motors.c#L716

However, some users have requested to be able to set this directly through a setpoint instead (like with roll/pitch/yaw). See old issue on this that was closed since we found out that rate control was possible: #572