buildman: Exit after reading toolchain

Recent refactoring changed buildman to continue operation after fetching
a toolchain. Fix this.

Fixes: b8680646521 ("bulidman: Move toolchain handling to a function")

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index 5c57200..f2ffb7f 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -610,6 +610,9 @@
     toolchains = get_toolchains(toolchains, col, args.override_toolchain,
                                 args.fetch_arch, args.list_tool_chains,
                                 args.verbose)
+    if isinstance(toolchains, int):
+        return toolchains
+
     output_dir = setup_output_dir(
         args.output_dir, args.work_in_output, args.branch,
         args.no_subdirs, col, clean_dir)