security settings - usb and developer mode

[stepansnigirev]

Now it's possible to change the following settings:

• USB communication. When disabled it doesn't even appear on a computer.
• Developer settings. When disabled micropython doesnt mount flash to the computer and REPL is also unavailable.

This settings screen is hidden under the PIN, so should be ok.

I decided to keep these settings as files because parsing jsons is errorprone. Plus boot.py should be as minimal as possible.