makefile: Fix building many different template rpm packages at the same time

Store build_timestamp for each built template to not mix them up in
update-repo-installer.

Makefile

@@ -15,7 +15,7 @@ help:
 
 
 rpms:
-	@echo $(TIMESTAMP) > build_timestamp
+	@echo $(TIMESTAMP) > build_timestamp_$(DIST)
 	@echo "Building template: $(TEMPLATE_NAME)"
 	@createrepo -q -g $$PWD/comps-qubes-template.xml yum_repo_qubes/$(DIST) -o yum_repo_qubes/$(DIST) && \
 	sudo -E ./fedorize_image fedorized_images/$(TEMPLATE_NAME).img $(PKGLISTFILE) && \

build_template_rpm

@@ -8,6 +8,7 @@ fi
 
 rpmbuild --target noarch \
          --define "template_name $NAME" \
+         --define "DIST $DIST" \
          --define "_topdir $PWD/rpmbuild" \
          --define "_tmppath $PWD/rpmbuild/tmp" \
          -bb templates.spec

templates.spec

@@ -4,7 +4,7 @@
 #
 
 %{!?version: %define version %(cat version)}
-%{!?rel: %define rel %(cat build_timestamp)}
+%{!?rel: %define rel %(cat build_timestamp_%{DIST})}
 
 Name:		qubes-template-%{template_name}
 Version:	%{version}