Add serial over BLE

[tannewt]

Also fixes 0-length writes meant to truncate a file.

[dhalbert]

The code looks good!

I am not sure how to test this. I loaded it on a Feather '840, and didn't see it advertising in the Bluefruit app or in the Web Bluetooth Dashboard.

[tannewt]

Boards with USB and BLE won't advertise unless they've got bonding info stored. If not, you need to press reset when the status led blinks fast and blue. That'll reset and then publicly advertise. The tx power is -40 so it needs to be close to the other device at that point too. Once it's connected, it goes back to 0 tx power.