support metamodule, remove built-in overlayfs mount

Co-authored-by: weishu <twsxtd@gmail.com>
Co-authored-by: YuKongA <70465933+YuKongA@users.noreply.github.com>
Co-authored-by: Ylarod <me@ylarod.cn>

userspace/meta-overlayfs/src/main.rs

@@ -0,0 +1,29 @@
+use anyhow::Result;
+use log::info;
+
+mod defs;
+mod mount;
+
+fn main() -> Result<()> {
+    // Initialize logger
+    env_logger::builder()
+        .filter_level(log::LevelFilter::Info)
+        .init();
+
+    info!("meta-overlayfs v{}", env!("CARGO_PKG_VERSION"));
+
+    // Dual-directory support: metadata + content
+    let metadata_dir = std::env::var("MODULE_METADATA_DIR")
+        .unwrap_or_else(|_| defs::MODULE_METADATA_DIR.to_string());
+    let content_dir = std::env::var("MODULE_CONTENT_DIR")
+        .unwrap_or_else(|_| defs::MODULE_CONTENT_DIR.to_string());
+
+    info!("Metadata directory: {}", metadata_dir);
+    info!("Content directory: {}", content_dir);
+
+    // Execute dual-directory mounting
+    mount::mount_modules_systemlessly(&metadata_dir, &content_dir)?;
+
+    info!("Mount completed successfully");
+    Ok(())
+}