convert RC PWM signals to 0-100% duty cycle for drive motor

[kiran-mohan]

The existing donkey code controls the drive motor using RC PWM signals which have a 20ms period and the signal range is between 1ms to 2ms. The car purchased has the RC controller's receiver and ESC integrated into one circuit, and so, it is difficult to command RC PWM signals to the native ESC / RC receiver of the car. Hence, a separate ESC is going to be used which would take regular PWM signals with a range of 0-100% duty cycle.

The output from the donkey driver needs to be scaled appropriately.

[griswaldbrooks]

@kiran-mohan updated a config.py which allowed the robot to scale appropriately.