System reinstall composefs

[Johan-Liebert1]

Add a composefs-backend option to system-reinstall-bootc

Fix a bug with installing grub bootloader where we were using paths absolute to rootfs even if /boot was a separate partition

[gemini-code-assist]

Code Review

This pull request adds a composefs-backend option for system reinstallation and fixes a bug in GRUB bootloader path generation when /boot is a separate partition. The changes are well-implemented across the configuration and command-line layers. The bug fix correctly determines the absolute path prefix, and the refactoring to use Utf8PathBuf::join improves code clarity and safety. I've included one suggestion to improve variable naming for better readability.

[gemini-code-assist]

For clarity and to avoid confusion, it would be better to rename the local variable entries_path. It currently has the same name as the entries_path field in the BLSEntryPath struct, but they represent different concepts. The local variable is a path prefix for the bootloader configuration, while the struct field is the directory where kernel/initrd files are written. Renaming it to something like abs_path_prefix would make the code easier to understand.

        Bootloader::Grub => {
            let root = Dir::open_ambient_dir(&root_path, ambient_authority())
                .context("Opening root path")?;

            // Grub wants the paths to be absolute against the mounted drive that the kernel +
            // initrd live in
            //
            // If "boot" is a partition, we want the paths to be absolute to "/"
            let abs_path_prefix = match root.is_mountpoint("boot")? {
                Some(true) => "/",
                // We can be fairly sure that the kernels we target support `statx`
                Some(false) | None => "/boot",
            };

            (
                BLSEntryPath {
                    entries_path: root_path.join("boot"),
                    config_path: root_path.join("boot"),
                    abs_entries_path: abs_path_prefix.into(),
                },
                None,
            )
        }