webkit: disable on SuperH

Webkit has been failing on SuperH since a long time, with nobody
caring to fix the problem. Since anyway our Webkit version is old and
in the process of being updated, our best option for 2014.08 is to
simply disallow Webkit on SuperH.

Fixes:

  http://autobuild.buildroot.org/results/3a0/3a0c48f65c06f7f19e608d54307c0788a0130589/
  http://autobuild.buildroot.org/?reason=webkit-1.11.5&arch=sh4a



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>