Function impl_completion 

pub(crate) async fn impl_completion(
    rootfs: &Dir,
    sysroot: &Sysroot,
    stateroot: Option<&str>,
) -> Result<()>

Core entrypoint for completion of an ostree-based install to a bootc one:

• kernel argument handling
• logically bound images

We could also do other things here, such as write an aleph file or ensure the repo config is synchronized, but these two are the most important for now.