Fix rpmtar target to use RELEASE variable instead of parsing spec file

The rpmtar target was extracting the version by grepping the spec file:
  grep "^%define version" pkg/rpm/sems.spec | awk '{print $3}'

After the spec file was changed to use %(cat VERSION) RPM macro syntax,
the grep/awk extracted the literal string "%(cat" instead of the actual
version, causing the tarball to be named "sems-%(cat.tar.gz".

Fix by using the existing RELEASE variable from Makefile.defs, which
already correctly reads and processes the VERSION file.

Author: marcel

Makefile

@@ -83,7 +83,7 @@ tar:
 # the tarball can be used for rpm building 
 .PHONY: rpmtar
 rpmtar: 
-	$(eval RPM_VERSION := $(shell grep "^%define version" pkg/rpm/sems.spec | awk '{print $$3}'))
+	$(eval RPM_VERSION := $(RELEASE))
 	rm -rf /tmp/_tar1
 	rm -rf /tmp/_tar2
 	rm -rf "~/rpmbuild/SOURCES/$(NAME)-*.tar.gz"