opkg: ensure image provided feeds are available after sysupgrade

Split the opkg.conf into three files, to make it easier to support custom
feeds and configs:

* /etc/opkg.conf -> base opkg configuration
* /etc/opkg/distfeeds.conf -> default Openwrt package feeds
* /etc/opkg/customfeeds.conf -> custom package feeds

Of these three, only the base opkg.conf and the customfeeds.conf is marked
as to be kept, so that the distfeeds.conf from the image is always used.

To ease migration, a script is added that moves any feeds from /etc/opkg.conf
to /etc/opkg/customfeeds.conf on first boot.

Also ensure that any keys used for verification are also kept in upgrade.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46491 3c298f89-4303-0410-b956-a3cf2f4a3e73