build: improve feed handling for opkg.conf

 - Consider not installed feeds as well
 - Add option to decide whether to comment disabled feeds

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42931 3c298f89-4303-0410-b956-a3cf2f4a3e73