feat: only send HID reports to one endpoint

Added some utility functions and an event for tracking the state of the
USB connection.

Updated endpoints.c to select a single endpoint to send HID reports to
based on the status of the USB and BLE connections. Partially fixes #206.

Future commits will add a user setting to control which endpoint is used if
both USB and BLE are ready.

app/include/zmk/events/usb-conn-state-changed.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <usb/usb_device.h>
+
+#include <zmk/event-manager.h>
+#include <zmk/usb.h>
+
+struct usb_conn_state_changed {
+    struct zmk_event_header header;
+    enum zmk_usb_conn_state conn_state;
+};
+
+ZMK_EVENT_DECLARE(usb_conn_state_changed);