Booting the live CD requires nomodeset on iMac with AMD/ATI Mobility Radeon HD 4670 [1002:9488]

When trying to boot the live CD of Ubuntu 16.04 Xenial Beta2 on an iMac with AMD/ATI Mobility Radeon HD 4670 [1002:9488] :

- With unchanged grub commands, I systematically get a black screen (even Ctrl-Alt-F1 provides NO readable console).

- With the 'nomodeset' parameter added to the 'linux' command of grub, the graphical display works OK.

I suggest that syslinux better checks the hardware and takes appropriate decisions automatically.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: syslinux 3:6.03+dfsg-11ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6
Uname: Linux 4.4.0-15-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CasperVersion: 1.368
CurrentDesktop: GNOME
Date: Wed Apr 6 21:14:43 2016
Dependencies:
 gcc-6-base 6-20160319-0ubuntu1
 libc6 2.23-0ubuntu2
 libgcc1 1:6-20160319-0ubuntu1
 mtools 4.0.18-2
 syslinux-common 3:6.03+dfsg-11ubuntu1
LiveMediaBuild: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Beta amd64 (20160323.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: syslinux
UpgradeStatus: No upgrade log present (probably fresh install)