opkg: don't run merger in case when /usr/lib/ipkg is link

opkg: don't run merger in case when /usr/lib/ipkg is link

* if there is link /usr/lib/ipkg -> /usr/lib/opkg, then merger will
  create backups of "both" alternatives directories, but they will be
  both the same /usr/lib/opkg/alternatives, then it will try to merge
  with output that every file is the same, sofar no real harm.
* Problem is after successfull merge, when it removes
  /usr/lib/ipkg/alternatives which is in this case the right
  alternatives in opkg :/
* If you've already this problem on your target then
*   # Restore old alternatives
*   rm -rf /usr/lib/opkg/alternatives
*   cp -ra /usr/lib/opkg/alternatives-backup /usr/lib/opkg/alternatives
*
*   # Regenerate links with right alternatives available
*   opkg install -force-reinstall busybox

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>