Skip relabel unless SELinux is enforcing

The SELinux relabel of the filesystem is taking almost 2 minutes and
isn't needed unless you actually plan to run with SELinux enforcing.
Plus, it appears to "leak" out of the chroot, referencing filesystems on
partitions that aren't even mounted in the chroot.

Note you just can't use getenforce or selinuxenabled here to get the
state of SELinux because those commands are not accurate inside a
chroot.

TBH, a downside of this is that if someone goes to try to enable SELinux
in an image where it was built with it not enabled, the file contexts
are going to be wrong. So they'd need to relabel themselves at that
point. However, this saves me quite a bit of time during image builds,
so I thought I'd submit to get other folks opinion on it.

Change-Id: I2132060d573fc93cf974f3560fdc651ff8ba38b4
Merge "Skip relabel unless SELinux is enforcing"