Make the /etc handling optional
Make the /etc handling optional

The gnome-sdk-image build need to actually write to /etc, so
we need to keep supporting it being a link to usr/etc.

handsome-feng/flatpak,flatpak/flatpak,alexlarsson/xdg-app,matthiasclasen/xdg-app,amigadave/flatpak,chergert/xdg-app,matthiasclasen/flatpak,amigadave/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,alexlarsson/xdg-app,flatpak/flatpak,amigadave/flatpak,handsome-feng/flatpak,alexlarsson/xdg-app,GeorgesStavracas/flatpak,thiblahute/xdg-app,matthiasclasen/flatpak,thiblahute/xdg-app,matthiasclasen/xdg-app,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,chergert/xdg-app,chergert/xdg-app,alexlarsson/xdg-app,thiblahute/xdg-app,matthiasclasen/xdg-app,amigadave/flatpak,chergert/xdg-app,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,matthiasclasen/xdg-app,thiblahute/xdg-app,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak