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.
Related: rhbz#1687743
commit 66241f7cd7 added a check on
runtime_img to create UDF iso's. Ends up it is only in outroot for live,
so switch all the checks to look at it in inroot instead.
Some images are becoming REALLY large. When a file is >= 4GiB we need to
pass -allow-limited-size to mkisofs to tell it to make a UDF image. Note
that the manpage says that this may result in it not booting on all
systems.
