message: [openwrt] fix startup script no effect
  The statement script needs to be on the first line, otherwise
  executing "/etc/init.d/otbr-agent start" has no effect.
diff: +#!/bin/sh /etc/rc.common
  #
  # Copyright (c) 2020, The OpenThread Authors.
  # All rights reserved.
  # POSSIBILITY OF SUCH DAMAGE.
  #
  -#!/bin/sh /etc/rc.common
  START=90
  