Wait for device to disappear after "fastboot reboot".

This is a follow-up to commit 157b0017.
After "fastboot reboot", "fastboot devices" was still showing the device as
connected even though it was starting to reboot; now "fastboot reboot" will
wait until the device has disconnected.

The wait timeout is 3 seconds.

Change-Id: I8066741f0be91e53f3e3f5bc968bc1b9de31db05