Development sandbox: Improve first experience with selection of MicroPython/Board

Hi there,

we received some reports from @ClemensGruber, @thiasB and @einsiedlerkrebs like [1] (thanks!) that it is unfortunate that the Pyboard-flavor (i.e. vanilla MicroPython) is used as the default when no `presets.mk` is present.

With kind regards,
Andreas.

[1] https://community.hiveeyes.org/t/einrichtung-von-terkin-unter-win10-wsl-firmware-entwicklungs-sandbox-datalogger/2110/11