As for the update system its fairly simple, the first thing the Link does is send a request to. http://media.steampowered.com/steamlink/06_2015/public_builds.txt
That returns a newline delimited list of the 'public' Steam Link builds. If there is a higher version in that list somewhere then it will send a request to http://media.steampowered.com/steamlink/06_2015/SystemUpdate_delta_[current_version]_[latest_version].zip
That will download the delta update and the system will reboot. Its actually pretty well thought out how they designed the update system, it uses bsdiff to keep the updates small.
Also after some trial and error I found that http://media.steampowered.com/steamlink/06_2015/SystemUpdate_full_[version].zip is also available for full updates, these zips even contain the recovery/bootloader files.