- Allow specifying an alternative board filter for the bootloader TTY,
as this will present different vendor, model, and serial than RIOT's
USB implementation.
- Allow specifying the serial. This is useful when multiple CDC ACM
devices are present.
Fix flashing with `MOST_RECENT_PORT=1` by detecting the board when
either in RIOT mode or in bootloader mode (which matches different
USB vendor and model strings).
Support for programming with the nrf52-u2f bootloader was dropped
apparently, but the documentation still recommends that over the
Arduino Bootloader. This brings doc and code in sync.
This is an Arduino Zero clone without the embedded debugger. Thus, it will be
flashed by default using the U2F bootloader and uses by default USB CDC ACM for
STDIO.
