feeds.conf.default: restore the old package feed temporarily

As explained here:
https://forum.openwrt.org/viewtopic.php?id=51078

allow both the old and the new packages feed to co-exist for a grace
period of 3 weeks, allowing maintainers to move packages progressively.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41149 3c298f89-4303-0410-b956-a3cf2f4a3e73