Makefile: remove generated boards.cfg within make distclean

Signed-off-by: Roger Meier <roger@bufferoverflow.ch>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index d5b8454..a555bd8 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -308,7 +308,7 @@
         Args:
             commit: Commit object that is being built
             brd: Board object that is being built
-            stage: Stage that we are at (distclean, config, build)
+            stage: Stage that we are at (mrproper, config, build)
             cwd: Directory where make should be run
             args: Arguments to pass to make
             kwargs: Arguments to pass to command.RunPipe()