busybox provides /sbin/sulogin util-linux provides /usr/sbin/sulogin I believe the right place is /sbin/sulogin. If busybox is disabled, 'single' user login does not work with the current placement of the util-linux file.
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=fec42d37a879c7616ad26640a4991fd9108790db