Are you sure about that? The rootdirectory is a ramdrive which is embedded in the kernel, and so it's brand new on each boot. The /opt symlink is created by the start script .../.PKG/Entware-ng/etc/init.d/Entware-ng, which doesn't exist, as you said.When I uninstall entware-ng and reboot, there is still a symlink /opt showing to the non existing folder under .../.PKG/Entware-ng/
When the package is not installed (or disabled) you can safely delete that symlink. It's automatically re-created by the start script (which will also install if it can't find /opt/etc/init.d/rc.unslung after creating the symlink.