Summary

This PR adds Zephyr support for Seeed Studio's XIAO BLE NRF52840 SENSE board.

It also provides a good example of a richer Zephyr port than the default, adding:

• Frozen modules (including asyncio, upysh, aioble, neopixel, and aiorepl).
• Enough MicroPython features to support using aioble (at least for the temp_sensor.py example)
• JSON, random, re, struct, etc.
• Support for my Zephyr ADC code (see PR zephyr: Add ADC support #17615)
• Mounted LFS2 filesystem using the 2MiB flash mounted as /flash

You can build it in the usual way:

$ cd ports/zephyr
$ west build -b xiao_ble/nrf52840/sense . 

This will produce a .uf2 file in build/zephyr/zephyr.uf2 that can be copied into the XIAO-SENSE USB drive that appears when you double-click the RESET button.

Testing

I have tested this on several of these boards, with and without my ADC (#17615) and zensor.Sensor.attr_set() (#17653) PRs.

I have run the aioble temp_sensor.py example but haven't done much more with the BLE.

Trade-offs and Alternatives

The build size is as follows:

Memory region         Used Size  Region Size  %age Used
           FLASH:      428864 B       788 KB     53.15%
             RAM:      165908 B       256 KB     63.29%
        IDT_LIST:          0 GB        32 KB      0.00%