17091: USBUS: Add URB support r=benpicco a=bergzand
### Contribution description
This commit adds support for URBs (USB Request/Response Blocks). These
allow for submitting multi-transfer sized buffers with USBUS handling
the individual usbdev xmits. Multiple URBs can be queued at once for a
single endpoint and USBUS will handle them in the order of submission.
OUT endpoint URBs must always consist of a whole number of full-sized
transfers (N x MaxEndpointSize). They will automatically finish after
the endpoint received a transfer less than the endpoint size.
IN endpoints can be arbitrary-sized and do not have to consist of a
whole number of full-sized transmissions. They support a flag to
indicate that the last transfer in the sequence must be less than a full
sized transfer (USBUS_URB_FLAG_AUTO_ZLP) and this adds a zero length
transfer at the end of the transmissions if the last transfer was equal
to the maximum transfer size.
URBs can be cancelled, but if the URB is already being processed it will
be cancelled after the current transmission within the URB is finished.
If it is still in the queue it will immediately be removed from the
queue.
### Testing procedure
- `tests/usbus_cdc_ecm` should still work. Testing one of the usbdev-supported platform should be sufficient here.
### Issues/PRs references
Needs #17064
18148: sys/flash_utils: helpers to store data in flash r=benpicco a=maribu
### Contribution description
This helpers that allow storing, accessing, and working with data in flash that works for both classical Harvard architectures (which do not map flash also into the address space) as well as modern Harvard architectures and von-Neumann architectures.
With this, `examples/default` again runs on the Arduino Uno / Nano. Since this board is still the "entry kit" for many people to embedded hardware, it would be nice to support it with our default example.
### Testing procedure
`examples/default` should run and work on ATmega boards (especially ATmega328P and ATmega32U4 based boards) as well on all other boards now.
### Issues/PRs references
None
Co-authored-by: Koen Zandberg <koen@bergzand.net>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
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