runners: bossac: Arduino bootloader reset on macOS

Fixes compatibility with the Arduino bootloader reset mechanism when
using `west flash` on Darwin hosts.

stty is used to set the serial port in the bossac runner to 1200 baud on
Arduino ATSAMD21 bootloaders. The `-F` argument to stty is a GNU
coreutils addition, and thus it is missing from Darwin's BSD stty, so
change the flag to `-f` when we're not running on Linux.

On Darwin, set DEFAULT_BOSSAC_PORT to None and ask the user to select
one from a list of IOCalloutDevices reported by ioreg, modelled on the
get_board_snr from the nrfjprog runner. This is because serial port TTYs
are generated by the device driver, and therefore there is no safe
default.

Tested with an Arduino Nano 33 IoT board.

Signed-off-by: George White <me@galexite.uk>