buildman: Add a test for --boards

Add a simple functional test for the --boards option. Fix the example in
the docs while we are here. Also improve the docs for Builder.count so it
is clearer what it contains.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.rst b/tools/buildman/buildman.rst
index a94957b..dcccae9 100644
--- a/tools/buildman/buildman.rst
+++ b/tools/buildman/buildman.rst
@@ -159,7 +159,7 @@
 
 .. code-block:: bash
 
-  buildman --boards sandbox,snow --boards
+  buildman --boards sandbox,snow --boards firefly-rk3399
 
 It is convenient to use the -n option to see what will be built based on
 the subset given. Use -v as well to get an actual list of boards.