Drop module image if no module remaining (#2447)

This fixes the issue that module image will always exist even if there
is no module to be loaded. Sadly we need to boot twice because we can
only know module status after image is mounted.

userspace/ksud_overlayfs/src/module.rs

@@ -281,6 +281,18 @@ pub fn prune_modules() -> Result<()> {
         Ok(())
     })?;
 
+    // collect remaining modules, if none, remove img
+    let remaining_modules: Vec<_> = std::fs::read_dir(defs::MODULE_DIR)?
+        .filter_map(|entry| entry.ok())
+        .filter(|entry| entry.path().join("module.prop").exists())
+        .collect();
+
+    if remaining_modules.is_empty() {
+        info!("no remaining modules, deleting image files.");
+        std::fs::remove_file(defs::MODULE_IMG).ok();
+        std::fs::remove_file(defs::MODULE_UPDATE_IMG).ok();
+    }
+
     Ok(())
 }