env-update: change prelink to use /etc/prelink.conf.d/portage.conf

Newer prelinks can support /etc/prelink.conf.d/ files.  So that prelink
can install /etc/prelink.conf and manage it itself, have env-update only
write /etc/prelink.conf.d/portage.conf instead of clobbering the main
/etc/prelink.conf file.

This should be backwards compatible as portage will conditionally change
/etc/prelink.conf to use the new /etc/prelink.conf.d/ too.

URL: http://bugs.gentoo.org/266855
Signed-off-by: Mike Frysinger <vapier@gentoo.org>