feat(boards): Add Kconfig for nrf52840dongle ZMK board variant (#3284)

To ensure we can tune things when other threads may have priority,
preventing UART processing in time for the studio UI requirements,
adjust our default UART thread priority, and allow overriding as needed.

app/boards/nordic/nrf52840dongle/Kconfig.nrf52840dongle

@@ -0,0 +1,5 @@
+# Copyright (c) 2026 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+config BOARD_NRF52840DONGLE
+    select ZMK_BOARD_COMPAT if BOARD_NRF52840DONGLE_NRF52840_ZMK

app/src/studio/Kconfig

@@ -59,11 +59,20 @@ config ZMK_STUDIO_TRANSPORT_UART
     select RING_BUFFER
     default y if $(dt_chosen_enabled,$(DT_CHOSEN_ZMK_STUDIO_RPC_UART))
 
+if ZMK_STUDIO_TRANSPORT_UART
+
 config ZMK_STUDIO_TRANSPORT_UART_RX_STACK_SIZE
     int "RX Stack Size"
     depends on !UART_INTERRUPT_DRIVEN
     default 512
 
+config ZMK_STUDIO_TRANSPORT_UART_RX_PRIORITY
+    int "RX Thread Priority"
+    depends on !UART_INTERRUPT_DRIVEN
+    default 9
+
+endif
+
 config ZMK_STUDIO_TRANSPORT_BLE
     bool "BLE (GATT)"
     select RING_BUFFER
@@ -71,9 +80,12 @@ config ZMK_STUDIO_TRANSPORT_BLE
     depends on ZMK_BLE
     default y
 
+
 config BT_CONN_TX_MAX
     default 64 if ZMK_STUDIO_TRANSPORT_BLE
 
+if ZMK_STUDIO_TRANSPORT_BLE
+
 config ZMK_STUDIO_TRANSPORT_BLE_PREF_LATENCY
     int "BLE Transport preferred latency"
     default 10
@@ -81,6 +93,8 @@ config ZMK_STUDIO_TRANSPORT_BLE_PREF_LATENCY
       When the studio UI is connected, a lower latency can be requested in order
       to make the interactions between keyboard and studio faster.
 
+endif
+
 endmenu
 
 config ZMK_STUDIO_RPC_THREAD_STACK_SIZE

app/src/studio/uart_rpc_transport.c

@@ -65,7 +65,7 @@ static void uart_rx_main(void) {
 }
 
 K_THREAD_DEFINE(uart_transport_read_thread, CONFIG_ZMK_STUDIO_TRANSPORT_UART_RX_STACK_SIZE,
-                uart_rx_main, NULL, NULL, NULL, K_LOWEST_APPLICATION_THREAD_PRIO, 0, 0);
+                uart_rx_main, NULL, NULL, NULL, CONFIG_ZMK_STUDIO_TRANSPORT_UART_RX_PRIORITY, 0, 0);
 
 #endif