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.

app/boards/kinesis/adv360pro/adv360pro.zmk.yml

@@ -13,5 +13,5 @@ outputs:
   - usb
   - ble
 siblings:
-  - adv360pro_left
-  - adv360pro_right
+  - adv360pro_left//zmk
+  - adv360pro_right//zmk

app/boards/kinesis/adv360pro/board.yml

@@ -3,7 +3,11 @@ boards:
     vendor: kinesis
     socs:
       - name: nrf52840
+        variants:
+          - name: zmk
   - name: adv360pro_right
     vendor: kinesis
     socs:
       - name: nrf52840
+        variants:
+          - name: zmk