ubifs: Add support for looking up directory and relative symlinks

This patch adds support for resolving symlinks to directories as well as
relative symlinks. Symlinks are now always resolved during file lookup,
so the load stage no longer needs to special-case them.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Stefan Roese <sr@denx.de>