refactor(core): Adjust our approach for upstream Zephyr boards (#3145)

refactor(core): Adjust our approach for upstream Zephyr boards

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`.

refactor(boards): Move to ZMK specific variants for the nRFMicro board

For consistency, adjust the nRFMicro board definition to offer a
"vanilla" Zephyr board, and then ZMK variants, e.g.
`nrfmicro/nrf52840/zmk`.

refactor(boards): Move to ZMK specific variant for the bluemicro840 board

For consistency, adjust the bluemicro840 board definition to offer a
"vanilla" Zephyr board, and then ZMK variant, e.g. `bluemicro840//zmk`.

refactor(boards): Make tofu65 to ZMK variant by default

Make the standard Tofu65 board ID be `tofu65/rp2040/zmk` or
`tofu65//zmk` by shorthand.

refactor(boards): Move BDN9 to ZMK variant by default

Make the standard BDN9 board ID be `bdn9/stm32f072xb/zmk` or
`bdn9//zmk` by shorthand.

refactor(boards): Move Puchi BLE to ZMK variant by default

Make the standard Puchi BLE board ID be `puchi_ble/nrf52840/zmk` or
`puchi_ble//zmk` by shorthand.

refactor(boards): Move Adv360 Pro to ZMK variant by default

Make the standard Adv360 Pro board ID be `adv360pro_left/nrf52840/zmk` or
`adv360pro_left//zmk` by shorthand and for right as well.

refactor(boards): Move nRF52840 M2 to ZMK variant by default

Make the standard nRF52840 M2 board ID be `nrf52840_m2/nrf52840/zmk` or
`nrf52840_m2//zmk` by shorthand.

refactor(boards): Move Pillbug to ZMK variant by default

Make the standard Pillbug board ID be `pillbug/nrf52840/zmk` or
`pillbug//zmk` by shorthand.

refactor(boards): Move s40nc to ZMK variant by default

Make the standard s40nc board ID be `s40nc/nrf52840/zmk` or
`s40nc//zmk` by shorthand.

refactor(boards): Move nice!60 to ZMK variant by default

Make the standard nice!60 board ID be `nice60/nrf52840/zmk` or
`nice60//zmk` by shorthand.

refactor(boards): Move planck to ZMK variant by default

Make the standard planck board ID be `planck/stm32f303xc/zmk` or
`planck//zmk` by shorthand.

refactor(boards): Move preonic to ZMK variant by default

Make the standard preonic board ID be `preonic/stm32f303xc/zmk` or
`preonic//zmk` by shorthand.

refactor(boards): Move ferris to ZMK variant by default

Make the standard ferris board ID be `ferris/stm32f072xb/zmk` or
`ferris//zmk` by shorthand.

refactor(boards): Move Proton-C to ZMK variant by default

Make the standard Proton-C board ID be `proton_c/stm32f303xc/zmk` or
`proton_c//zmk` by shorthand.

refactor(boards): Move Corneish Zen to ZMK variant by default

Make the standard Corneish Zen board ID be `corneish_zen_left/nrf52840/zmk` or
`corneish_zen_left//zmk` by shorthand and for right as well.

refactor(boards): Move nice!nano to ZMK variant by default

Make the standard nice!nano board ID be `nice_nano/nrf52840/zmk` or
`nice_nano//zmk` by shorthand.

refactor(boards): Move mikoto to ZMK variant by default

Make the standard mikoto board ID be `mikoto/nrf52840/zmk` or
`mikoto//zmk` by shorthand.

refactor(boards): Move Polarity Works boards to ZMK variants

Make the standard Polarity Works board IDs be `zmk` variants.

doc: Update docs/blog post to reference ZMK variants

* Update Zephyr 4.1 blog post to mention ZMK variants
* Add note to hardware support page about variants

docs: Fix up shield board overlays for new board IDs

Adjust our documentation to properly use the correct qualified board
overlay file names that match our new board conventions.

app/boards/jpconstantineau/bluemicro840/board.yml

@@ -1,5 +1,5 @@
 board:
-  name: bluemicro840
-  vendor: jpconstantineau
-  socs:
-    - name: nrf52840
+  extend: bluemicro840
+  variants:
+    - name: zmk
+      qualifier: nrf52840