buildman: Use yellow consistently for warning lines

At present warnings are shown in yellow in the summary (-s) but magenta in
the detail listing (-e). Use yellow in both.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index 6211476..01d8bf4 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -1279,7 +1279,7 @@
             _OutputErrLines(better_err, colour=self.col.GREEN)
             _OutputErrLines(worse_err, colour=self.col.RED)
             _OutputErrLines(better_warn, colour=self.col.CYAN)
-            _OutputErrLines(worse_warn, colour=self.col.MAGENTA)
+            _OutputErrLines(worse_warn, colour=self.col.YELLOW)
 
         if show_sizes:
             self.PrintSizeSummary(board_selected, board_dict, show_detail,