Have firmware log battery voltage in excess nfc space

[ckemere]

A timestamp and battery voltage will allow us to accurately estimate battery life, though it will require logging to get the data off.

[ckemere]

This is implemented but not tested in recent firmware version. The challenge is that android by default will write ndef short records if payload is small, so firmware has to check and potentially update essentially the whole record with each write rather than just the incremental update.