Check that /srv/tftpboot/suse-11.3/install is not a symlink

Using a symlink is not supported for PXE boot, unfortunately.

https://bugzilla.novell.com/show_bug.cgi?id=886196
Merge pull request #2049 from vuntz/no-symlink

Check that /srv/tftpboot/suse-11.3/install is not a symlink