Adds support for autoupdating portage with portage images

src/platform/dev/autoupdate.py

Index: src/platform/dev/autoupdate.py
diff --git a/src/platform/dev/autoupdate.py b/src/platform/dev/autoupdate.py
index 2f0fe956b543fcd3af230c68e789b46a49b7d41d..f3b74479fe628d5ce4b9c647ef2cdad9a0a8756b 100644
--- a/src/platform/dev/autoupdate.py
+++ b/src/platform/dev/autoupdate.py
@@ -47,6 +47,9 @@ class Autoupdate(BuildObject):
 
   def GetLatestVersion(self, latest_image_path):
     latest_version = latest_image_path.split('/')[-1]
+
+    # Removes the portage build prefix.
+    latest_version = latest_version.lstrip("g-")
     return latest_version.split('-')[0]
 
   def CanUpdate(self, client_version, latest_version):