ubnuntu 18.04 boot timeout

Step：

1. cd etc/systemd/system/network-online.target.wants/systemd-networkd-wait-online.service
2. nano systemd-networkd-wait-online.service
3. add "-s -q --timeout=2" at the end of line "ExecStart=/lib/systemd/systemd-networkd-wait-online"
4. reboot