This removes a number of FIXMEs regarding driver dependencies on the
main code.
The code to verify pulse lengths is now in computeRC which means that
all RX drivers do not have to duplicate the pulse length checking code.
This means that failsafe can be used to validate serial RX providers as
well as PWM/PPM RX providers.
By decoupling everything the structures now only contain members they need. The mapping code is simplified. The calculation of timer periods is now where it belongs (with the output code, not with the mapping code). Also, since each motor output has it's own callback method it is technically possible to mix brushed and brushless motors if the brushed motors and brushless motors use different timers. Additional code would be required to fully support that.
