build: trigger pacakge index creation for all feeds if required

When CONFIG_PER_FEED_REPO_ADD_DISABLED is set, trigger index creation for
any available feed, regardless of whether there where binaries built or not.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45765 3c298f89-4303-0410-b956-a3cf2f4a3e73