Mount selinuxfs when other filesystems are mounted

Be consistent when mounting filesystems, and mount selinuxfs
at the same time other filesystems are mounted. In particular,
this ensures that a /sys/fs/selinux/null is available at early
boot, avoiding an unnecessary mknod call.

Change-Id: I01e6b3900f48b4cb3f12d8a928e1e95911524252