scripts/feeds: allow export of feeds with static revisions

Add a switch to scripts/feeds that allows listing the feeds with their
currently checked out revisions in feeds.conf compatible format.

This allows providing a feeds.conf for public builds to make replication
of the build easier.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46706 3c298f89-4303-0410-b956-a3cf2f4a3e73