SECURITY_SELINUX_DISABLE should be enable on X s390x

== SRU Justification ==
Security team requires the CONFIG_SECURITY_SELINUX_DISABLE should be
enabled in all of our kernels.

Currently it's not enabled for s390x in Xenial. And causing the
test_081_config_security_selinux_disable test in ubuntu_kernel_security
test suite complaining about this:

  ======================================================================
  FAIL: test_081_config_security_selinux_disable (__main__.KernelSecurityConfigTest)
  Ensure CONFIG_SECURITY_SELINUX_DISABLE is disabled (LP: #1680315)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "./test-kernel-security.py", line 2158, in test_081_config_security_selinux_disable
      self.assertKernelConfig('SECURITY_SELINUX_DISABLE', expected)
    File "./test-kernel-security.py", line 207, in assertKernelConfig
      self.assertKernelConfigSet(name)
    File "./test-kernel-security.py", line 194, in assertKernelConfigSet
      '%s option was expected to be set in the kernel config' % name)
  AssertionError: SECURITY_SELINUX_DISABLE option was expected to be set in the kernel config

== Test ==
A test kernel could be found here:
https://people.canonical.com/~phlin/kernel/lp-1813721-s390x-selinux/

This issue can be verified with a q-r-t test:
test_081_config_security_selinux_disable, the test will pass with the
patched kernel.

  test_081_config_security_selinux_disable (__main__.KernelSecurityConfigTest)
  Ensure CONFIG_SECURITY_SELINUX_DISABLE is disabled (LP: #1680315) ... (skipped: l) ok

== Regression Potential ==
Low, we already have this config enabled in all kernels except this
specific Xenial s390x.

  ----------------------------------------------------------------------

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-142-generic 4.4.0-142.168
ProcVersionSignature: Ubuntu 4.4.0-142.168-generic 4.4.167
Uname: Linux 4.4.0-142-generic s390x
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
AlsaDevices: Error: command ['ls', '-l', '/dev/snd/'] failed with exit code 2: ls: cannot access '/dev/snd/': No such file or directory
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: s390x
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
CurrentDmesg:

Date: Tue Jan 29 02:30:42 2019
HibernationDevice: RESUME=UUID=ca468a9c-9563-442c-85c6-6055e800a66e
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lspci:

Lsusb: Error: command ['lsusb'] failed with exit code 1:
PciMultimedia:

ProcFB: Error: [Errno 2] No such file or directory: '/proc/fb'
ProcKernelCmdLine: root=UUID=b65b756a-ba4e-4c53-aa32-0db2bdb50bb3 crashkernel=196M
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-142-generic N/A
 linux-backports-modules-4.4.0-142-generic N/A
 linux-firmware 1.157.21
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)