feat(boards): Add Arduino Uno interconnect, nrf52840dk config.

app/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.zmk.yml

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+file_format: "1"
+id: nrf52840dk_nrf52840
+name: Nordic nRF52840 DK
+type: board
+arch: arm
+outputs:
+  - usb
+  - ble
+url: https://www.nordicsemi.com/Products/Development-hardware/nrf52840-dk
+exposes: [arduino_uno]

app/boards/interconnects/arduino_uno/arduino_uno.zmk.yml

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+file_format: "1"
+id: arduino_uno
+name: Arduino Uno Rev3
+type: interconnect
+url: https://store-usa.arduino.cc/products/arduino-uno-rev3?selectedStore=us
+manufacturer: Arduino
+description: |
+  The Arduino Uno Rev3 is a board who's popularity lead to countless shields being developed for it. By
+  natural extension, once there were many shields designed for it, many other *boards* began to be developed
+  that were compatible to leverage the extensive available shields. Today, many dev kits come with Uno
+  headers to make it easy to work with them.
+
+  Note: ZMK doesn't support boards with AVR 8-bit processors, such as the ATmega32U4, because Zephyr™ only
+  supports 32-bit and 64-bit platforms. As a result, boards like the original Arduino Uno Rev3 itself are
+  *not* supported by ZMK.

app/boards/nrf52840dk_nrf52840.conf

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+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+CONFIG_ZMK_USB=y
+CONFIG_ZMK_BLE=y
+
+CONFIG_MPU_ALLOW_FLASH_WRITE=y
+CONFIG_NVS=y
+CONFIG_SETTINGS_NVS=y
+CONFIG_FLASH=y
+CONFIG_FLASH_PAGE_LAYOUT=y
+CONFIG_FLASH_MAP=y