Add PRESUBMIT.py for infra/config

This adds a PRESUBMIT.py file for infra/config, which works with `git cl
presubmit` from depot_tools. The presubmit function checks that the
lucicfg generated output matches the changes to main.star and has no
validation errors.

Fixes the existing config file to make it conform to the presubmit
checks (e.g. formatting, docstrings).

Also adds a mention of depot_tools to CONTRIBUTING.md to make it clear
that depot_tools works on the boringssl repo.

Change-Id: I311cab911b80025db5c0d780f8959b58c033d81c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80627
Auto-Submit: Lily Chen <chlily@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

infra/config/PRESUBMIT.py

@@ -0,0 +1,24 @@
+# Copyright 2025 The BoringSSL Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Enforces consistency in lucicfg definitions for BoringSSL's CI and CQ.
+
+Run by the presubmit API in depot_tools, e.g. by running `git cl presubmit`.
+"""
+
+PRESUBMIT_VERSION = '2.0.0'
+
+def CheckLucicfgGenOutputMain(input_api, output_api):
+  return input_api.RunTests(input_api.canned_checks.CheckLucicfgGenOutput(
+      input_api, output_api, 'main.star'))