Fixed tftpboot symlink check

As it is possible to use autofs to mount the medias we need to set
symlinks to the autofs root folder. Now the condition checks if the
symlink is set within the tftpboot root, if it is everything is fine, if
it points outside of the root the installation stops with an error
message.
Merge pull request #2063 from tboerger/feature/symlink-check

Fixed tftpboot symlink check