When receiving a `QUIT` message, the main thread was trying to tell
the scanner thread to quit - however, what it was actually doing was
creating a local variable that shadowed the global flag used by the
scanner thread. Fix that by ensuring that the main thread uses the
global variable instead.
Fixes#1028
Allow the IDE to detect UF2 volumes (i.e. when you hold BOOTDEL and
plug in the board).
Allows the IDE2 to properly upload using OTA and serial.
Fixes#890 and others