This adds a new livemedia-creator argument, --extra-boot-args, which can
be used to add arguments to the kernel command-line in the templates.
(cherry picked from commit 235813212f)
xorrisofs needs to use iso-level 3 when creating images with large
files. This adds tests to all the relevant templates, just like we used
to do for udf and mkisofs.
(cherry picked from commit 2264496fe85cedae77af35b509b140a5811d5743)