Added a generic path_lookup method in vfs

Added a generic path_lookup method in vfs

Well, for now, just applied to EXTLINUX. Here is the main change:
1) Add to union structure in file to make the older path_lookup method work.
2) Add a generic inode structure, to represent one file.
3) Add three more methods in fs_ops; they are:
	iget_root, get the root inode of a fs
	iget_current, get the 'pwd'
	iget, do_open a file
4) Add a *TEMP* memory managemant system.

Signed-off-by: Yuanhan Liu <Aleaxander@gmail.com>