为什么 Centos 7 内核更新未标记为安全更新？

Question

我有一个 Centos 7.2 机器。如果我运行“yum check-update --security”命令，我会得到...

...
--> kernel-3.10.0-514.2.2.el7.x86_64 from updates excluded (updateinfo)
--> kernel-tools-3.10.0-514.2.2.el7.x86_64 from updates excluded (updateinfo)
--> kernel-tools-libs-3.10.0-514.2.2.el7.x86_64 from updates excluded (updateinfo)
...
1 package(s) needed for security, out of 196 available

epel-release.noarch                      7-9                              extras
python2-rsa.noarch                       3.4.1-1.el7                      epel

为什么 kernel-* 软件包没有被标记为安全更新？我该怎么做才能包含它们？

这是我的 /etc/yum.conf 文件

etc]# cat yum.conf 
[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=5
bugtracker_url=http://bugs.centos.org/set_project.php?project_id=23&ref=http://bugs.centos.org/bug_report_page.php?category=yum
distroverpkg=centos-release