Add ability to have board overlay combined w/ shield keymap.

app/cmake/zmk_config.cmake

@@ -44,6 +44,9 @@ endif()
 set(CACHED_ZMK_CONFIG ${ZMK_CONFIG} CACHE STRING "Selected user ZMK config")
 
 foreach(root ${BOARD_ROOT})
+	if (EXISTS "${root}/boards/${BOARD}.overlay")
+		list(APPEND ZMK_DTC_FILES "${root}/boards/${BOARD}.overlay")
+	endif()
 	find_path(BOARD_DIR
 	    NAMES ${BOARD}_defconfig
 	    PATHS ${root}/boards/*/*

app/dts/common/arduino_uno_pro_micro_map.dtsi

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2020 Pete Johanson
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/* This provies a mapping from Arduino Uno to Arduino Pro Micro pins for development */
+
+/ {
+	pro_micro_d: connector_d {
+		compatible = "arduino-pro-micro";
+		#gpio-cells = <2>;
+		gpio-map-mask = <0xffffffff 0xffffffc0>;
+		gpio-map-pass-thru = <0 0x3f>;
+		gpio-map
+			= <0 0 &arduino_header 6 0>		/* D0 */
+			, <1 0 &arduino_header 7 0>		/* D1 */
+			, <2 0 &arduino_header 8 0>		/* D2 */
+			, <3 0 &arduino_header 9 0>		/* D3 */
+			, <4 0 &arduino_header 10 0>		/* D4/A6 */
+			, <5 0 &arduino_header 11 0>		/* D5 */
+			, <6 0 &arduino_header 12 0>		/* D6/A7 */
+			, <7 0 &arduino_header 13 0>		/* D7 */
+			, <8 0 &arduino_header 14 0>		/* D8/A8 */
+			, <9 0 &arduino_header 15 0>		/* D9/A9 */
+			, <10 0 &arduino_header 16 0>		/* D10/A10 */
+			, <16 0 &arduino_header 17 0>	/* D16 */
+			, <14 0 &arduino_header 18 0>	/* D14 */
+			, <15 0 &arduino_header 19 0>	/* D15 */
+			;
+	};
+
+	pro_micro_a: connector_a {
+		compatible = "arduino-pro-micro";
+		#gpio-cells = <2>;
+		gpio-map-mask = <0xffffffff 0xffffffc0>;
+		gpio-map-pass-thru = <0 0x3f>;
+		gpio-map
+			= <0 0 &arduino_header 0 0>	/* A0 */
+			, <1 0 &arduino_header 1 0>	/* A1 */
+			, <2 0 &arduino_header 2 0>	/* A2 */
+			, <3 0 &arduino_header 3 0>	/* A3 */
+			;
+	};
+};
+
+pro_micro_i2c: &arduino_i2c {};
+pro_micro_spi: &arduino_spi {};
+pro_micro_serial: &arduino_serial {};