Summary

I need to redirect some of the TinyUSB callbacks, but MicroPython implements them exclusively. So far I've been using linker --wrap with great success, sadly TinyUSB 0.19.0 adds weak symbols for callbacks which breaks the --wrap trick.
This patch adds MICROPY_TINYUSB_CALLBACK macro that by default expands to the callback name unchanged. Users can define this macro to add a prefix (e.g., mp_) to callback implementations, to redirect or completely override MicroPython's TinyUSB callbacks.

Other changes

Add hook to mp_usbd_task. This adds an empty hook that gets called when running the USB task.

Testing

MICROPY_TINYUSB_CALLBACK macro expands to the callback name unchanged, I've double checked the map file and in both cases (default, or defined to mp_) the symbols have the right names. Let me know if more testing needed.

Trade-offs and Alternatives

This is the most direct way I could think of, however:

• --wrap no longer works with TinyUSB 0.19.0.
• objcopy to rename symbols. I had a hard time trying to get this to work, it requires complex rules somewhere, but might be a viable option if this is closed.
• Hooks?

  // In mp_usbd_cdc.c
  void tud_cdc_rx_cb(uint8_t itf) {
      // Call user hook if registered
      if (mp_usbd_cdc_rx_hook != NULL) {
          if (mp_usbd_cdc_rx_hook(itf)) {
              return; // Hook handled it
          }
      }
      // Default implementation
      mp_usbd_cdc_rx_cb_default(itf);
  }

• MICROPY_EXCLUDE_SHARED_TINYUSB_USBD_CDC seems to have been needed at some point, but it requires a full implementation of those callbacks. I only need to redirect/override some of them.

• Make mp callbacks weak?
  1- Rename those callbacks to mp_tud_cdc_x
  2- Implement tud_cdc_ as yet another weak symbol that calls mp_tud_cdc_x
  3- Now we can provide strong symbols that override them, and call mp_tud_cdc_x if needed.

This depends on the link order, I think the shared MP objects always come first though.