install: explicitly install osk-sdl when needed

This adds osk-sdl to the rootfs when --fde is set, or when building a rootfs for the ondev installer. Ideally the ondev installer would selectively install osk-sdl if the user opted for fde at runtime, but I haven't found a straight forward way to enable that yet, and this behavior here is no different than the current behavior (where osk-sdl is always installed in the rootfs by way of depends= in pmos-mkinitfs).

For images that are built without --fde, osk-sdl won't be installed at all in the rootfs, once the dependency is dropped from pmos-mkinitfs in https://gitlab.com/postmarketOS/pmaports/-/merge_requests/2242