New design:
* capture output to a vstr
* compare the complete vstr to boot_out.txt
* rewrite if not a complete match
This is resilient against future changes to the automatic
text written to boot_out.txt.
This also fixes rewriting boot_out.txt in the case where
boot.py prints something.
Perhaps it also saves a bit of code space. Some tricks:
* no need to close a file in read mode
* no need to switch on/off USB write access, going down to the
oofatfs layer doesn't check it anyway
This started while adding USB MIDI support (and descriptor support is
in this change.) When seeing that I'd have to implement the MIDI class
logic twice, once for atmel-samd and once for nrf, I decided to refactor
the USB stack so its shared across ports. This has led to a number of
changes that remove items from the ports folder and move them into
supervisor.
Furthermore, we had external SPI flash support for nrf pending so I
factored out the connection between the usb stack and the flash API as
well. This PR also includes the QSPI support for nRF.
