feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

app/src/behaviors/behavior_endpoints.c

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_behavior_endpoints
+
+#include <device.h>
+#include <devicetree.h>
+#include <drivers/behavior.h>
+
+#include <dt-bindings/zmk/endpoints.h>
+
+#include <zmk/behavior.h>
+#include <zmk/endpoints.h>
+
+#include <logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+static int on_keymap_binding_pressed(struct zmk_behavior_binding *binding,
+                                     struct zmk_behavior_binding_event event) {
+    switch (binding->param1) {
+    case ENDPOINT_TOGGLE_CMD:
+        return zmk_endpoints_toggle();
+    case ENDPOINT_USB_CMD:
+        return zmk_endpoints_select(ZMK_ENDPOINT_USB);
+    case ENDPOINT_BLE_CMD:
+        return zmk_endpoints_select(ZMK_ENDPOINT_BLE);
+    default:
+        LOG_ERR("Unknown endpoints command: %d", binding->param1);
+    }
+
+    return -ENOTSUP;
+}
+
+static int behavior_ep_init(struct device *dev) { return 0; }
+
+static const struct behavior_driver_api behavior_endpoints_driver_api = {
+    .binding_pressed = on_keymap_binding_pressed,
+};
+
+DEVICE_AND_API_INIT(behavior_end, DT_INST_LABEL(0), behavior_ep_init, NULL, NULL, APPLICATION,
+                    CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &behavior_endpoints_driver_api);