[ipkg] Generate gzipped package lists

 - Can be used by changing the src line in /etc/ipkg.conf to src/gz
 - Cannot be used by busybox ipkg

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@11981 3c298f89-4303-0410-b956-a3cf2f4a3e73