Add option to specify that router should download the Download Master packages from the Internet when installing it, but consider that other apps such as AiCloud and cloudsync are already built-in.  This allows to save 8 MB of space on the AC56/AC68 firmware
Added (unsupported) build profile and files for RT-N18U.  Untested.
webui: Treat the RT-AC56S the same as an RT-AC56U/R; report CPU temp for RT-N18U (untested)
stealth mode: Add (untested) support for RT-AC56S