WIP: pybd-sf2: add the Pyboard D series SF2 variant

[aykevl]

Product page: https://store.micropython.org/product/PYBD-SF2-W4F2

Definitely not finished: blinky1 doesn't even work correctly (it only seems to turn on the red LED). If you want to use it, first put the device in DFU mode. Then run the following commands (assuming the MicroPython repository was cloned in ~/src/micropython):

tinygo build -o test.elf -target=pybd-sf2 -size=short examples/blinky1
arm-none-eabi-objcopy -O binary test.elf test.bin
~/src/micropython/tools/dfu.py -b 0x08008000:test.bin test.dfu
~/src/micropython/tools/pydfu.py -u test.dfu

EDIT: forgot to mark this PR as a draft.