[ipkg] Port the full ipkg to Kamikaze

 - Does not suffer from most of the issues the busybox implementation experiences
 - busybox ipkg is still the default package manager
 - Removes /usr/bin/ipkg on install if it is a symlink
 - Relinks /usr/bin/ipkg to busybox on removal if ipkg is compiled into busybox
 - Supports gzip'd package lists, results in substantial space savings

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@11980 3c298f89-4303-0410-b956-a3cf2f4a3e73