To not clutter the system when building an opkg free image, generate the
distfeeds.conf only if CLEAN_IPKG is unset.
Since opkg is now a shared package, we can't rely on PACKAGE_opkg, but
since opkg is not reasonably usable without the status information, we
can tie the distfeeds.conf to it.
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>