it is more convenient to provide the URL via make option rather than
rely on hardcoded value in the code base. it provides more flexibility
in case of different servers for stable, daily, canary, community, ...
and instead of parsing a file in the filesystem to obtain target name of
the device on which lakka is running, we can provide the target name at
build time as well. the target name is used for the full path URL for
the download of updates.
there is no need to have these values editable via retroarch frontend
and saved in the config file. it is safer to provide them at build time
and have them set this way.
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