buildman: Use oldconfig when adjusting the config

We cannot be sure that the new config is consistent, particularly when
changing a major item like CONFIG_CMDLINE. Use 'make oldconfig' to
check that and avoid any such problems.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/func_test.py b/tools/buildman/func_test.py
index 55dd494..6b88ed8 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -439,6 +439,8 @@
             tools.write_file(fname, b'CONFIG_SOMETHING=1')
             return command.CommandResult(return_code=0,
                     combined='Test configuration complete')
+        elif stage == 'oldconfig':
+            return command.CommandResult(return_code=0)
         elif stage == 'build':
             stderr = ''
             fname = os.path.join(cwd or '', out_dir, 'u-boot')
@@ -461,7 +463,7 @@
             return command.CommandResult(return_code=0)
 
         # Not handled, so abort
-        print('make', stage)
+        print('_HandleMake failure: make', stage)
         sys.exit(1)
 
     # Example function to print output lines