build: introduce per feed repository support

This changeset implements a new menuconfig option to generate separate
repositories for each enabled package feed instead of one monolithic one.

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

Backport of r42002

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42016 3c298f89-4303-0410-b956-a3cf2f4a3e73