Move to using proper HWMv2 board extensions
https://docs.zephyrproject.org/4.1.0/hardware/porting/board_porting.html#board-extensions
for extending upstream Zephyr boards for use with ZMK. With this change,
using upstream Zephyr board IDs directly, e.g. `seeeduino_xiao` will be
stock versions as found upstream. To use a board variant that is tuned
for ZMK use, use the `zmk` variant, e.g. `seeeduino_xiao//zmk` which is
shorthand for `seeeduino_xiao/samd21g18a/zmk`.
Peter Johanson