When logging directly to files (the not-using-memlogd case) the onboot
services must log to /run/log because /var/log might be overmounted
by a persistent disk. Therefore we create a symlink at the end of
the onboot section.
When logging via memlogd, all logs are buffered until a logwrite service
starts, so no symlink is needed.
Signed-off-by: David Scott <dave.scott@docker.com>