Before this commit, livecd-tools used "drivers+=" in the dracut
configuration file which can be used when creating the initrd for the
Live images. As a result, the default set of drivers dracut knows about
was not included into the initrd, only the specified ones were.
Broken ISO images are a likely result of that.
livecd-tools was patched to use "add_drivers+=" instead.
Note that not all (if any) build scripts for the rosa2016.1 ISOs use the
dracut config generated by livecd-tools, so this fix is not that
important. Still, worth applying to simplify future maintenance.
