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.
Resolves: rhbz#1689314
live media isn't exactly the same as the Anaconda install media. Right
now this amounts to needing a root= cmdline argument but in the future
there may be other differences.
This also reverts 5437557846 on the new copies of the templates.
