composefs-backend: Implement bootc usr-overlay

[Johan-Liebert1]

Similar to ostree, mount a transient overlayfs on /usr

[gemini-code-assist]

Code Review

This pull request implements the bootc usr-overlay command for the composefs backend, which creates a transient writable overlay on /usr. The changes look good and follow the pattern of other commands. I've identified a couple of areas for improvement: a typo in a new public function name, a more robust way to find the root mount point, and a simplification of some conditional compilation logic to improve maintainability. Overall, a solid addition.

[gemini-code-assist]

The conditional compilation logic here is a bit redundant. The else branch for when composefs-backend is enabled is identical to the case where it's disabled. You can simplify this to improve readability and reduce duplication.

        Opt::UsrOverlay => {
            #[cfg(feature = "composefs-backend")]
            if composefs_booted()?.is_some() {
                return composefs_usr_overlay();
            }
            usroverlay().await
        }

[cgwalters]

Thanks for doing this!