NFR: Add battery power management

[Eric-FR]

Current hardware with 5 V connection between adalogger and F9x will not power the F9x while running on battery connected to the adalogger. One should instead connects them trough 3.3 V. Indeed, this is not stable. One have to add a decoupling capacitor. Adding a 47 µF capacitor seems to give stable operation. Further tests are needed for confirmation.

To avoid corruption of SD card, recording should be stopped when voltage is dropping. In fact, when stopping writing on the SD card, the voltage is rising slightly back. So, a small gap should be set between switch off and restart. But if power is back on USB, it should restart immediately even if battery voltage is still low. A detection of USB should be added by connecting the USB pin to an input through a voltage divider.

[Eric-FR]

A power switch must added with connection to En(able) pin of the adalogger to switch off the board while battery is completely discharged and/or is under USB charge.