Add Boardsource 3x4 shield

app/boards/shields/boardsource3x4/boardsource3x4.overlay

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <dt-bindings/zmk/matrix-transform.h>
+
+/ {
+    chosen {
+        zmk,kscan = &kscan0;
+    };
+
+    kscan0: kscan {
+        compatible = "zmk,kscan-gpio-matrix";
+        label = "KSCAN";
+        diode-direction = "col2row";
+
+         row-gpios
+            = <&pro_micro_a 0 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)>
+            , <&pro_micro_a 1 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)>
+            , <&pro_micro_a 2 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)>
+            ;
+
+        col-gpios
+            = <&pro_micro_d 10 GPIO_ACTIVE_HIGH>
+            , <&pro_micro_d 16 GPIO_ACTIVE_HIGH>
+            , <&pro_micro_d 14 GPIO_ACTIVE_HIGH>
+            , <&pro_micro_d 15 GPIO_ACTIVE_HIGH>
+            ;
+
+    };
+
+    bt_unpair_combo: bt_unpair_combo {
+		compatible = "zmk,bt-unpair-combo";
+		key-positions = <0 11>;
+	};
+};