common/config: make SELinux protection conditional

common/config: make SELinux protection conditional

When selinux utilities are present, xfstests add options to help
circumvent selinux protection. However, on Chrome OS, it leads to
the opposite effect, when it prevents mount to succeed.

BUG=chromium:669641
TEST=xfstest test ext4/001 completes where it use to display:
common/rc: could not mount /dev/loop29 on /usr/local/autotest/tmp/xfstests_TEST

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>