download: do nothing on update if we're sure that the source hasn't changed

slapos/recipe/download.py

@@ -76,4 +76,6 @@ class Recipe(object):
         shutil.copyfileobj(fsrc, fdst)
     return result
 
-  update = install
+  def update(self):
+    if not self._md5sum:
+      self.install()