ARDrone2 RAW battery readout

Hello Paparazzi devs,

This PR contains an implementation for battery readout for ARDrone2. It works, but can still be improved by improving the formula for transforming 10bit adc value for voltage range of 0-1.5V to the ardrone battery voltage. I dont know how the battery is electrically connected so I'm not sure on how to properly improve this yet. One idea is to measure a few fully charged battery and a few depleted ones and write down the max and min voltage. then with the same batteries measure the 10bit adc value and solve a function for it. 

For now this can be used, but I cant officially say much about the accuracy.

Regards,

Dino