buildman: Show the build rate at the end

It is interesting to note the number of builds completed per second to
track machine performance and build speed. Add a 'rate' value at the end
of the build to show this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index f2756ea..dbb75b3 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -1677,7 +1677,8 @@
             if duration.microseconds >= 500000:
                 duration = duration + timedelta(seconds=1)
             duration = duration - timedelta(microseconds=duration.microseconds)
-            msg += ', duration %s' % duration
+            rate = float(self.count) / duration.total_seconds()
+            msg += ', duration %s, rate %1.2f' % (duration, rate)
         Print(msg)
 
         return (self.fail, self.warned)