boards/seeedstudio-xiao-nrf52840: initial board support

[mguetschow]

Contribution description

Initial board support for the tiny Seed Studio XIAO nRF52840. It uses the same bootloader as the Adafruit nrf52840 Feather (Sense) boards.

Testing procedure

• I'm still in the process of running BUILD_IN_DOCKER=1 dist/tools/compile_and_test_for_board/compile_and_test_for_board.py . seeedstudio-xiao-nrf52840 locally and will report back with the result later.

• Look through the header and configuration files. I've mostly followed the example from feather-nrf52840-sense and nrf52840dk (for the external flash).

• Check generated documentation for the board.

[mguetschow]

• I'm still in the process of running BUILD_IN_DOCKER=1 dist/tools/compile_and_test_for_board/compile_and_test_for_board.py . seeedstudio-xiao-nrf52840 locally and will report back with the result later.

Currently failing tests:

$ find results -name "*.failed"
results/seeedstudio-xiao-nrf52840/tests/sys/congure_test/test.failed
results/seeedstudio-xiao-nrf52840/tests/sys/pthread_flood/test.failed
results/seeedstudio-xiao-nrf52840/tests/sys/congure_quic/test.failed
results/seeedstudio-xiao-nrf52840/tests/sys/usbus/test.failed
results/seeedstudio-xiao-nrf52840/tests/sys/shell/test.failed
results/seeedstudio-xiao-nrf52840/tests/core/thread_msg/test.failed
results/seeedstudio-xiao-nrf52840/tests/core/msg_queue_print/test.failed
results/seeedstudio-xiao-nrf52840/tests/riotboot/test.failed
results/seeedstudio-xiao-nrf52840/tests/turo/test.failed
results/seeedstudio-xiao-nrf52840/tests/cpu/mpu_noexec_ram/test.failed
results/seeedstudio-xiao-nrf52840/tests/cpu/mpu_stack_guard/test.failed
results/seeedstudio-xiao-nrf52840/tests/riotboot_hdr/test.failed
results/seeedstudio-xiao-nrf52840/tests/pkg/libschc/test.failed

[benpicco]

Most of those are software-only and won't depend on the board.
compile_and_test_for_board.py isn't the greatest way to test a board, there are tests/periph/spi, tests/periph/uart and tests/drivers/mtd_raw that come with a test command but that requires RX/TX / MISO/MOSI to be connected or a writable MTD device (in case the board exposes MTD). ``tests/periph/i2calso includesi2c_scan`, but that can be included everywhere with `shell_cmd_i2c_scan`.

(Thinking of it, we should probably add a general board test app that makes it easier to evaluate a newly ported board)

[mguetschow]

Most of those are software-only and won't depend on the board.

True, but I've still managed to find some issues which are at least board-related :)

there are tests/periph/spi, tests/periph/uart and tests/drivers/mtd_raw

Great, I'll have a look! Do we have a list of such useful test applications for new boards somewhere? May be good to add a pointer to the documentation.

(Thinking of it, we should probably add a general board test app that makes it easier to evaluate a newly ported board)

Or even better!