ubifs: Correct dereferencing of files-after-symlinks

Files in directories which are symlinked to were not dereferenced
correctly in last commit. E.g., with a symlink

   /boot/lnk -> /boot/real_dir

loading

   /boot/lnk/uImage

will fail. This patch fixes that by simply seeing to it that the target
base directory has a slash after it.

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