Currently the sysroot file is named 'sysroot'. This prevents to have
different latests sysroot for different architectures. For example, ade
sysroot latest --arch arm64 returns the url of the armhf sysroot if it
was the last sysroot uploaded.
A symbolic link called sysroot can be used so that older version of
ade are still able to download a sysroot file.

Apertis: https://phabricator.apertis.org/T3849



Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Reviewed-by: Simon McVittie <smcv@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6678