[buildiso] deletes image-folders on re-run
Created by: manjaro
Seems clears workdir on re-run. Within ManjaroISO we have the option to remove for example xfce-image and call buildiso once again. As root-image was already created it skips that step and continues with xfce-image. This feature is missing within manjaro-tools. Therefore we have to add an option to disable workdir clearing.