postmarketos-mkinitfs: don't depend on osk-sdl, add to initfs conditionally

This replaces the dependency on osk-sdl with a dependency on a postmarketos-fde-unlocker virt. package

osk-sdl and its dependencies are also added contitionally to the initfs, based on whether the osk-sdl exe is installed in the rootfs

Also see: pmbootstrap!2066 (merged)