mc_pos_control: replace takeoff velocity ramp with thrust ramp

The velocity ramp had problems with:
- different vehicle tunings resulted in the start value of the resulting
thrust ramp staring either higher and lower than zero thrust.
lower -> delay of beginning
higher -> small jump at beginning
- when a task set position and velocity at the same time during takeoff
(which AutoSmoothVel does) it resulted in a velocity setpoint
jump at the end of the ramp because the additional velocity
setpoint correction from the position controller was not considered.

The thrust ramp should now be very deterministic:
- always start at zero
- always end at the curreant thrust setpoint output
of the complete position controller