roomservice: use revision tag instead of branch

Change-Id: I100fa4f9616208e63729e77e07f7ff5759f4c823
diff --git a/build/tools/roomservice.py b/build/tools/roomservice.py
index e57af5a..13f9db6 100755
--- a/build/tools/roomservice.py
+++ b/build/tools/roomservice.py
@@ -219,8 +219,8 @@
             project = ElementTree.Element("project", attrib = { "path": repo_target,
                 "remote": "github", "name": "BlissRoms-Devices/%s" % repo_name })
 
-        if 'branch' in repository:
-            project.set('revision',repository['branch'])
+        if 'revision' in repository:
+            project.set('revision',repository['revision'])
         elif fallback_branch:
             print("Using fallback branch %s for %s" % (fallback_branch, repo_name))
             project.set('revision', fallback_branch)