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/src/power.c

@@ -23,14 +23,7 @@ static u32_t power_last_uptime;
 
 bool is_usb_power_present() {
 #ifdef CONFIG_USB
-    enum usb_dc_status_code usb_status = zmk_usb_get_status();
-    switch (usb_status) {
-    case USB_DC_DISCONNECTED:
-    case USB_DC_UNKNOWN:
-        return false;
-    default:
-        return true;
-    }
+    return zmk_usb_is_powered();
 #else
     return false;
 #endif /* CONFIG_USB */