Enable the bugfix-12146-syslinux-fixes APT overlay (refs: #12146).

Changes in that overlay:

syslinux (3:6.03+dfsg-14.1+tails1) bugfix-12146-syslinux-fixes; urgency=medium

  * Cherry-pick 3 upstream commits that might fix isohybrid issues
    (refs: Tails#12146).

 -- intrigeri <intrigeri@boum.org>  Fri, 26 May 2017 08:47:28 +0000