Cleanup and simplify

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>

extras/vagrant/build.sh

@@ -67,6 +67,6 @@ elif [ "$OS_ID" == "centos" ]; then
     (cd $VPP_DIR/vnet ;$SUDOCMD aclocal;$SUDOCMD automake -a)
     $SUDOCMD make pkg-rpm
 elif [ "$OS_ID" == "opensuse" ]; then
-    $SUDOCMD make build-release
+    $SUDOCMD make pkg-rpm
 fi