buildman: Deal more nicely with invalid build-status file

The 'done' files created by buildman may end up being empty if buildman
runs out of disk space while writing them. At present buildman dies with
an exception when using -s to check the build status. Fix this.

Seriesl-cc: trini

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builder.py

@@ -673,7 +673,12 @@ class Builder:
         environment = {}
         if os.path.exists(done_file):
             with open(done_file, 'r') as fd:
-                return_code = int(fd.readline())
+                try:
+                    return_code = int(fd.readline())
+                except ValueError:
+                    # The file may be empty due to running out of disk space.
+                    # Try a rebuild
+                    return_code = 1
                 err_lines = []
                 err_file = self.GetErrFile(commit_upto, target)
                 if os.path.exists(err_file):