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.

app/boards/makerdiary/nrf52840_m2/Kconfig.nrf52840_m2

@@ -3,6 +3,6 @@
 
 config BOARD_NRF52840_M2
     select SOC_NRF52840_QIAA
-    imply RETAINED_MEM
-    imply RETENTION
-    imply RETENTION_BOOT_MODE
+    imply RETAINED_MEM if BOARD_NRF52840_M2_NRF52840_ZMK
+    imply RETENTION if BOARD_NRF52840_M2_NRF52840_ZMK
+    imply RETENTION_BOOT_MODE if BOARD_NRF52840_M2_NRF52840_ZMK

app/boards/makerdiary/nrf52840_m2/board.yml

@@ -1,5 +1,5 @@
 board:
-  name: nrf52840_m2
-  vendor: makerdiary
-  socs:
-    - name: nrf52840
+  extend: nrf52840_m2
+  variants:
+    - name: zmk
+      qualifier: nrf52840

app/boards/makerdiary/nrf52840_m2/nrf52840_m2.zmk.yml

@@ -1,5 +1,5 @@
 file_format: "1"
-id: nrf52840_m2
+id: nrf52840_m2//zmk
 name: nRF52840 M.2 Module
 type: board
 arch: arm

app/boards/makerdiary/nrf52840_m2/nrf52840_m2_nrf52840_zmk.dts

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2026 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/dts-v1/;
+#include <../boards/makerdiary/nrf52840_m2/nrf52840_m2.dts>
+#include <common/nordic/nrf52840_uf2_boot_mode.dtsi>
+
+/ {
+    model = "Makerdiary nRF52840 M.2 module";
+    compatible = "makerdiary,nrf52840_m2";
+
+    chosen {
+        zmk,battery = &vbatt;
+    };
+
+    vbatt: vbatt {
+        compatible = "zmk,battery-voltage-divider";
+        io-channels = <&adc 0>;
+        output-ohms = <1000000>;
+        full-ohms = <(1000000 + 1000000)>;
+    };
+};

app/core-coverage.yml

@@ -13,7 +13,7 @@ include:
   - board: nice60
   - board: xiao_ble//zmk
     shield: hummingbird
-  - board: nrf52840_m2
+  - board: nrf52840_m2//zmk
     shield: m60
   - board: planck
   - board: proton_c

app/module/boards/makerdiary/nrf52840_m2/Kconfig.nrf52840_m2

@@ -0,0 +1,5 @@
+# Copyright (c) 2026 The ZMK Contributors
+# SPDX-License-Identifier: Apache-2.0
+
+config BOARD_NRF52840_M2
+    select SOC_NRF52840_QIAA

app/module/boards/makerdiary/nrf52840_m2/board.cmake

@@ -0,0 +1,7 @@
+# Copyright (c) 2026 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+board_runner_args(nrfjprog "--nrf-family=NRF52" "--softreset")
+
+include(${ZEPHYR_BASE}/boards/common/nrfjprog.board.cmake)
+include(${ZEPHYR_BASE}/boards/common/blackmagicprobe.board.cmake)

app/module/boards/makerdiary/nrf52840_m2/board.yml

@@ -0,0 +1,5 @@
+board:
+  name: nrf52840_m2
+  vendor: makerdiary
+  socs:
+    - name: nrf52840

app/module/boards/makerdiary/nrf52840_m2/nrf52840_m2.dts

@@ -6,7 +6,6 @@
 
 /dts-v1/;
 #include <nordic/nrf52840_qiaa.dtsi>
-#include <common/nordic/nrf52840_uf2_boot_mode.dtsi>
 
 / {
     model = "Makerdiary nRF52840 M.2 module";
@@ -16,7 +15,6 @@
         zephyr,code-partition = &code_partition;
         zephyr,sram = &sram0;
         zephyr,flash = &flash0;
-        zmk,battery = &vbatt;
     };
 
     leds {
@@ -31,14 +29,6 @@
             gpios = <&gpio0 31 GPIO_ACTIVE_HIGH>;
         };
     };
-
-    vbatt: vbatt {
-        compatible = "zmk,battery-voltage-divider";
-        io-channels = <&adc 0>;
-        output-ohms = <1000000>;
-        full-ohms = <(1000000 + 1000000)>;
-    };
-
 };
 
 &adc {

app/module/boards/makerdiary/nrf52840_m2/nrf52840_m2.yaml

@@ -0,0 +1,15 @@
+identifier: nrf52840_m2
+name: Makerdiary nRF52840 M.2 module
+type: mcu
+arch: arm
+toolchain:
+  - zephyr
+  - gnuarmemb
+  - xtools
+supported:
+  - adc
+  - usb_device
+  - ble
+  - ieee802154
+  - pwm
+  - watchdog

app/module/boards/makerdiary/nrf52840_m2/nrf52840_m2_defconfig

@@ -0,0 +1,12 @@
+# Copyright (c) 2026 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+# Enable MPU
+CONFIG_ARM_MPU=y
+
+# enable GPIO
+CONFIG_GPIO=y
+
+CONFIG_USE_DT_CODE_PARTITION=y
+CONFIG_BUILD_OUTPUT_UF2=y
+

app/module/boards/makerdiary/nrf52840_m2/pre_dt_board.cmake

@@ -0,0 +1,9 @@
+#
+# Copyright (c) 2026 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+#
+
+# Suppresses duplicate unit-address warning at build time for power, clock, acl and flash-controller
+# https://docs.zephyrproject.org/latest/build/dts/intro-input-output.html
+
+list(APPEND EXTRA_DTC_FLAGS "-Wno-unique_unit_address_if_enabled")