Only allow alphanumeric in flavor.

pungi

@@ -175,6 +175,10 @@ if __name__ == '__main__':
             parser.print_help()
             sys.exit(0)
             
+        if not config.get('default', 'flavor').isalnum():
+            print >> sys.stderr, "Flavor must be alphanumeric."
+            sys.exit(1)
+
         if opts.do_gather or opts.do_createrepo or opts.do_buildinstall or opts.do_createiso:
             opts.do_all = False
         return (opts, args)