readImageState method
Reads which images are currently present on the device.
Implementation
Future<ImageState> readImageState(Duration timeout) {
  return execute(
    Message(
      op: Operation.read,
      group: _imgGroup,
      id: _imgCmdState,
      flags: 0,
      data: CborMap({}),
    ),
    timeout,
  ).unwrap().then((value) => ImageState(value.data));
}