Switching default packages feed to github

New feed: https://github.com/openwrt/packages
Maintainers please move your packages there.

See also: https://lists.openwrt.org/pipermail/openwrt-devel/2014-June/025810.html

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41063 3c298f89-4303-0410-b956-a3cf2f4a3e73