appstream: Create symlinks for the icons/$origin subdirectory
appstream: Create symlinks for the icons/$origin subdirectory

For compatibility with libappstream we create a $origin ("flatpak")
subdirectory with symlinks to the size directories thus matching the
standard merged appstream layout if we assume the appstream has
origin=flatpak, which flatpak-builder creates.

See https://github.com/ximion/appstream/pull/224 for details.

Fixes https://github.com/flatpak/flatpak/pull/2779

Closes: #2789
Approved by: alexlarsson

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak