Introduce BootVolume abstraction class

BootVolume is initialized from a root directory of a volume. It finds
the system directory, and -- not implemented yet -- mounts the system
package, if the system is packaged, replacing the system directory with
it. Adjusted several functionality (main(), the loader functions,
user_menu()) to use BootVolume instead of the root directory.

headers/private/kernel/boot/vfs.h

@@ -99,11 +99,41 @@ class MemoryDisk : public Node {
 };
 
 
+class BootVolume {
+public:
+								BootVolume();
+								~BootVolume();
+
+		status_t				SetTo(Directory* rootDirectory);
+		void					Unset();
+
+		bool					IsValid() const
+									{ return fRootDirectory != NULL; }
+
+		Directory*				RootDirectory() const
+									{ return fRootDirectory; }
+		Directory*				SystemDirectory() const
+									{ return fSystemDirectory; }
+		bool					IsPackaged() const
+									{ return fPackaged; }
+
+private:
+			Directory*			fRootDirectory;
+				// root directory of the volume
+			Directory*			fSystemDirectory;
+				// "system" directory of the volume; if packaged the root
+				// directory of the mounted packagefs
+			bool				fPackaged;
+				// indicates whether the boot volume's system is packaged
+};
+
+
 /* function prototypes */
 
 extern status_t vfs_init(stage2_args *args);
 extern status_t register_boot_file_system(Directory *directory);
-extern Directory *get_boot_file_system(stage2_args *args);
+extern status_t get_boot_file_system(stage2_args* args,
+			BootVolume& _bootVolume);
 extern status_t mount_file_systems(stage2_args *args);
 extern int open_node(Node *node, int mode);
 extern int open_from(Directory *directory, const char *path, int mode,