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

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

app/boards/keebio/bdn9/bdn9.zmk.yml

@@ -1,5 +1,5 @@
 file_format: "1"
-id: bdn9
+id: bdn9//zmk
 name: BDN9 (Rev2)
 type: board
 arch: arm

app/boards/keebio/bdn9/board.yml

@@ -3,6 +3,8 @@ board:
   vendor: keebio
   socs:
     - name: stm32f072xb
+      variants:
+        - name: zmk
   revision:
     format: major.minor.patch
     default: 2.0.0

app/core-coverage.yml

@@ -9,7 +9,7 @@ shield:
   - settings_reset
   - tidbit
 include:
-  - board: bdn9
+  - board: bdn9//zmk
   - board: nice60
   - board: xiao_ble//zmk
     shield: hummingbird
@@ -40,7 +40,7 @@ include:
   - board: nice_nano
     shield: lily58_left nice_view_adapter nice_view
     nickname: "niceview"
-  - board: bdn9
+  - board: bdn9//zmk
     snippet: studio-rpc-usb-uart
     cmake-args: "-DCONFIG_ZMK_STUDIO=y"
     nickname: "stm32-studio"