Mirror the libretiny/ltchiptool shape for RP2040 serial when
--prebuilt-dir is set: open the user-supplied serial port at 1200 baud
(arduino-pico's USB CDC interprets the open/close as a request to reboot
into BOOTSEL), poll picotool until the BOOTSEL device shows up on the
USB bus, then dispatch to upload_using_picotool with the prebuilt .uf2.
This removes the last path that required --prebuilt-dir to contain a
platformio.ini + .pioenvs/<name>/ tree, so upload_using_platformio is
now only invoked when no prebuilt dir is set (i.e. the existing
compile+upload flow on a developer machine).
Issue: esphome/device-builder#572