build: enable PER_FEED_REPO by default

Enable the separate feed repositories by default to align custom builds with the repositories
available on the snapshot download server.

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

Backport of r42112

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42352 3c298f89-4303-0410-b956-a3cf2f4a3e73