This patch addresses two things:
1) It simplifies the Makefile to build RPMs by removing duplication and
use environment variables instead;
2) It aligns openSUSE with others with regards to invoke "pkg-rpm" by
the build.sh for Vagrant
Change-Id: I51557f54e4ddf49d83bb69fa0035c8fff122857e
Signed-off-by: Marco Varlese <marco.varlese@suse.com>