BATTERY_STATUS message: fixed validation.

- Previously could not validate BATTERY_STATUS message due to the field name "function". This caused an error when running generate.sh, unless the validation libraries were unavailable.
- Field name renamed to "battery_function", other fields renamed to be similar.
- Validation now passes.

More info:
https://groups.google.com/forum/#!topic/drones-discuss/Fonsr_lUR80