buildman: Avoid rebuilding when --mrproper is used

When this flag is enabled, 'make mrproper' is always used when
reconfiguring, so there is no point in doing it again.

Update this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index 0a7ff2e..c0b1067 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -700,7 +700,7 @@
                         job.work_in_output, job.adjust_cfg)
                 failed = result.return_code or result.stderr
                 did_config = do_config
-                if failed and not do_config:
+                if failed and not do_config and not self.mrproper:
                     # If our incremental build failed, try building again
                     # with a reconfig.
                     if self.builder.force_config_on_failure: