Update installation guide for syslinux 6
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

pshchelo/ironic,debayanray/ironic_backup,pshchelo/ironic,supermari0/ironic,dims/ironic,ramineni/myironic,dims/ironic,NaohiroTamura/ironic,hpproliant/ironic,ionutbalutoiu/ironic,rackerlabs/ironic,froyobin/ironic,openstack/ironic,NaohiroTamura/ironic,bacaldwell/ironic,devananda/ironic,naterh/ironic,SauloAislan/ironic,redhat-openstack/ironic,SauloAislan/ironic,rdo-management/ironic,Tehsmash/ironic,Tan0/ironic,bacaldwell/ironic,faizan-barmawer/openstack_ironic,naototty/vagrant-lxc-ironic,naototty/vagrant-lxc-ironic,ionutbalutoiu/ironic,openstack/ironic