Set default RELEASE variable

If builder.conf is generated by setup script, it is set, but otherwise
it isn't. And because of this, various scripts guess release version
based on repository dir name. Lets have it in one place.

QubesOS/qubes-issues#3935

Makefile

@@ -22,6 +22,9 @@ VERBOSE ?= 0
 COMPONENTS ?= builder
 
 LINUX_REPO_BASEDIR ?= $(SRC_DIR)/linux-yum/current-release
+# set default RELEASE based on LINUX_REPO_BASEDIR, assuming it was set to
+# something sensible (not the value above)
+RELEASE ?= $(patsubst r%,%,$(lastword $(subst /, ,$(LINUX_REPO_BASEDIR))))
 INSTALLER_COMPONENT ?= installer-qubes-os
 BACKEND_VMM ?= xen
 KEYRING_DIR_GIT ?= $(BUILDER_DIR)/keyrings/git
@@ -808,7 +811,7 @@ post-update-repo-%:
 	done; \
 	for repo in `echo $$repos_to_update|tr ' ' '\n'|sort|uniq`; do \
 		[ -z "$$repo" ] && continue; \
-		(cd $$repo/.. && ./update_repo-$*.sh `basename $$repo`); \
+		(cd $$repo/.. && ./update_repo-$*.sh r$(RELEASE)); \
 	done
 
 template-name: