[opensuse-buildservice] [PATCH] use pigz with kiwi if it is available in buildroot

Attached patch defines kiwi option --gzip-cmd to pigz if package available in buildroot. Otherwise gzip will be used (by default). pigz, which stands for parallel implementation of gzip, is a fully functional replacement for gzip that exploits multiple processors and multiple cores to the hilt when compressing data. My experience with pigz is ~6Min (gzip) vs ~2Min with pigz. Comments are welcome.