The concept is quite simple: stick a file on the OTA server named
something like `koreader-appimage-latest-stable` (by analogy with
`koreader-cervantes-latest-stable.zsync`), which contains nothing
but a filename.
The difference with the zsync update is that the link is then launched
in the user's browser (AppImage) or DownloadManager (Android, not yet
implemented).
* Android hasOTAUpdate = no for the moment
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