remove system archive feature

Just compress the initrd with zstd.
Remove rsmount dependency, mtab is easy enough to parse.

src/cmd/init/bootstrap.rs

@@ -180,8 +180,8 @@ async fn mount_system(cfg: &dkl::Config, bs_dir: &str, verifier: &Verifier) {
     for layer in &cfg.layers {
         let src = retry(async || {
             if layer == "modules" {
-                (fs::read("/modules.sqfs").await)
-                    .map_err(|e| format_err!("read /modules.sqfs failed: {e}"))
+                let src = "/modules.sqfs";
+                (fs::read(src).await).map_err(|e| format_err!("read {src} failed: {e}"))
             } else {
                 verifier.verify_path(&format!("{bs_dir}/{layer}.fs")).await
             }