sys-kernel: enable selinux support

Note, this enables SELinux automatically, in "permissive" mode, so things will still work properly.  You need to then "disable" SELinux in the boot sequence to keep things "fast".  We can't do it the other way around (default to off, and then enable after booting) as SELinux will not work that way (selinuxfs is not present, other things do not work, etc.)  This is by design...

Only merge this if you really want this, it should obsolete pull request #642