This explicit zero length xfer leads to a second CSW packet. If
another read was started between the two CSWs then the host gets
confused and resets the device.
On reads, the CSW is automatically sent when we reply with the
total length. Writes must do it manually so they can wait for the
disk to flush the data.
* 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
