title: adb wait-for-devices timeout/nonblocking?
body: <p>Is there a way to have a timeout when calling "adb wait-for-devices"?</p>
  
  <p>Scenario:</p>
  
  <pre><code>adb reboot
  adb wait-for-devices (timeout listed here - if possible)
  **if times out: echo timed out**
  **else**
  adb root
  adb wait-for-devices
  </code></pre>