Add console kernel parameters to extlinux configuration

Those parameters allow to access the console for VMs using
extlinux (CentOS 6 for example). They are already added to
Fedora images.

Change-Id: I035170bd101062e004fb3782b5f8e5a7944a4f03
Closes-Bug: 1296899
Merge "Add console kernel parameters to extlinux configuration"