This introduces a new spi_flash_api.h that works for both SPI and
QSPI. The previous spi_flash functions are now called external_flash
to minimize confusion.
* 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
