Explicitly clean the mock chroot before building.

Experienced a situation where mock failed to build the packages
because systemd-nspawn failed.
Cleaning the chroots before rebuild (both regular and bootstrap)
fixed the problem. Adding commands to do this explicitly during
the build process.

rpm/buildrpm.sh

@@ -84,6 +84,8 @@ do
 	echo "--------------------------------------------------------"
 	echo "  Mock-building packages for $config"
 	echo "--------------------------------------------------------"
+	mock --root=$config --quiet --clean
+	mock --bootstrap-chroot --root=$config --quiet --clean
 	if ! mock --bootstrap-chroot --root=$config --quiet --rebuild $srpm; then
 		echo "Mock build failed for $config."
 		echo "------------ build.log -------------"