docs: selinux: add '=' signs to kernel boot options

Provide the full kernel boot option string (with ending '=' sign).
They won't work without that and that is how other boot options are
listed.

If used without an '=' sign (as listed here), they cause an "Unknown
parameters" message and are added to init's argument strings,
polluting them.

  Unknown kernel command line parameters "enforcing checkreqprot
    BOOT_IMAGE=/boot/bzImage-517rc6", will be passed to user space.

 Run /sbin/init as init process
   with arguments:
     /sbin/init
     enforcing
     checkreqprot
   with environment:
     HOME=/
     TERM=linux
     BOOT_IMAGE=/boot/bzImage-517rc6

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: selinux@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
[PM: removed bogus 'Fixes' line]
Signed-off-by: Paul Moore <paul@paul-moore.com>