binman: Detect when valid images are not produced

When external blobs are missing, show a message indicating that the images
are not functional.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/tout.py b/tools/patman/tout.py
index 91a53f4..3330526 100644
--- a/tools/patman/tout.py
+++ b/tools/patman/tout.py
@@ -171,6 +171,7 @@
 
     # TODO(sjg): Move this into Chromite libraries when we have them
     stdout_is_tty = hasattr(sys.stdout, 'isatty') and sys.stdout.isatty()
+    stderr_is_tty = hasattr(sys.stderr, 'isatty') and sys.stderr.isatty()
 
 def Uninit():
     ClearProgress()