Wait for device to disappear after "fastboot reboot".

This is a follow-up to commit 157b00171a.
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

fastboot/fastboot.c

@@ -1216,6 +1216,7 @@ int main(int argc, char **argv)
     }
     if (wants_reboot) {
         fb_queue_reboot();
+        fb_queue_wait_for_disconnect();
     } else if (wants_reboot_bootloader) {
         fb_queue_command("reboot-bootloader", "rebooting into bootloader");
         fb_queue_wait_for_disconnect();