Also, re-enable calibration storage for CircuitPlayground Express.
Tested with a 500hz PWMOut on Metro M0 with Saleae:
* with crystal 500hz
* with usb 500hz +- 0.1hz
* without either 487hz += 0.1hz
SAMD51 is skipped due to DFLL errata and the fact it defaults to a
factory calibrated 48mhz that works fine for USB.
Fixes#648
.. the price of this appears to be about 112 bytes of flash and 12
bytes of RAM, according to the stats printed during the build.
It also uses up 4 directory entries (out of 128), but does not reduce
the number of blocks usable for storing file contents.
These are the same items noted in the Adafruit README for Trinket M0
as preventing MacOS indexing.
Closes: #689
* Introduce a python script to generate the USB descriptor instead of
a bunch of C macros. In the future, we can use this dynamically in
CircuitPython.
* Add support for detecting read-only mass storage mounts.
Fixes#377
