buildman: Allow showing the list of boards with -n

As well as showing the number of boards, allow showing the actual list of
boards that would be built, if -v is provided.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index ccea13f..f665276 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -141,7 +141,7 @@
 with 'ball'.
 
 It is convenient to use the -n option to see what will be built based on
-the subset given.
+the subset given. Use -v as well to get an actual list of boards.
 
 Buildman does not store intermediate object files. It optionally copies
 the binary output into a directory when a build is successful. Size