base-files: enable force_link for lan by default

This allows services to bind to it at boot time instead of waiting for a
cable to be plugged in

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40773 3c298f89-4303-0410-b956-a3cf2f4a3e73