Crystal not used? about uf2-samdx1

[dhalbert]

The bootloader clocks work well enough without using the crystal, especially since we can do clock recovery from the USB signals. Having just one way to do the clocks reduces the amount of code we need to maintain. Any application program loaded or started by the bootloader is free to reconfigure the clocks to use the crystal. Arduino programs and CircuitPython do use the crystal if available.