fs: ext4: Add support for the creation of symbolic links

Re-use the functions used to write/create a file, to support creation of a
symbolic link.
The difference with a regular file are small:
- The inode mode is flagged with S_IFLNK instead of S_IFREG
- The ext2_dirent's filetype is FILETYPE_SYMLINK instead of FILETYPE_REG
- Instead of storing the content of a file in allocated blocks, the path
to the target is stored. And if the target's path is short enough, no block
is allocated and the target's path is stored in ext2_inode.b.symlink

As with regulars files, if a file/symlink with the same name exits, it is
unlinked first and then re-created.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Fix ext4 env code]
Signed-off-by: Tom Rini <trini@konsulko.com>

include/ext4fs.h

@@ -135,9 +135,10 @@ int ext4fs_init(void);
 void ext4fs_deinit(void);
 int ext4fs_filename_unlink(char *filename);
 int ext4fs_write(const char *fname, const char *buffer,
-		 unsigned long sizebytes);
+				 unsigned long sizebytes, int type);
 int ext4_write_file(const char *filename, void *buf, loff_t offset, loff_t len,
 		    loff_t *actwrite);
+int ext4fs_create_link(const char *target, const char *fname);
 #endif
 
 struct ext_filesystem *get_fs(void);