for-15.05 luci-mod-admin-full: restore opkg feed config capability

for-15.05 luci-mod-admin-full: restore opkg feed config capability

opkg config change from r46491 was backported to CC15.05
by https://dev.openwrt.org/changeset/46848/

Existing /etc/opkg.conf was split to three:
   /etc/opkg.conf -> base opkg configuration
   /etc/opkg/distfeeds.conf -> default Openwrt package feeds
   /etc/opkg/customfeeds.conf -> custom package feeds

Since then the actual feed definitions have not been visible/configurable,
as only /etc/opkg.conf has been visible in Luci.

This patch restores the capability to see and edit package feed definitions.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry picked from commit a42c0bb6182fdde7591d1f1b163a9a3354b45f5a)