Update file util.sh

At some point in time - possibly when changes were made to make Gnome Boxes work with the ISO - the tools stopped honoring the config variable dist_name.

The dist_name was originally used as the leading word in the ISO filename - but the introduction of get_osid made the dist_name hardcoded to Manjaro.

Adding this patch to init_buildiso() function will restore this functionality. Custom ISO can then distinguesh themselves by not using Manjaro as part of the filename.

If the variables dist_name and iso_name is empty - function defaults to get_distname and get_osid

The proposed changes are as follows in file util.sh function init_buildiso()

    [[ -z ${dist_name} ]] && dist_name=$(get_distname)

    [[ -z ${iso_name}]] && iso_name=$(get_osid)

@romangg