Update installation guide for syslinux 6

Fedora 21 and the next Ubuntu will have syslinux 6 in them. In syslinux
6 modules are not "stand alone" as it was in versions prior to 4, see:
http://www.syslinux.org/wiki/index.php/Library_modules

Partial-Bug: #1351010
Change-Id: Ie6bb031c10a9f8cf978e2c5dbd643b4a19cc93d0
Merge "Update installation guide for syslinux 6"