.github: add github action to automate PR handling

This patch updates the Buildroot repository's GitHub configuration to better
manage incoming pull requests.

- Added a new GitHub Actions workflow (`repo-lockdown.yml`) to automatically
  handle new pull requests.

The new workflow:
- Triggers on new pull requests.
- Uses the `dessant/repo-lockdown` action to:
  - Comment on the pull request, guiding contributors to use the mailing list
    for patch submission.
  - Lock the pull request to prevent further discussion.
  - Close the pull request.

This change ensures that contributors are properly directed to the preferred
method of patch submission via the mailing list, maintaining consistency and
streamlining the review process.

Signed-off-by: Dongdong Zhang <zhangdongdong@eswincomputing.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

.github/workflows/repo-lockdown.yml

@@ -0,0 +1,25 @@
+name: 'Repo Lockdown'
+
+on:
+  pull_request_target:
+    types: opened
+
+permissions:
+  pull-requests: write
+
+jobs:
+  action:
+    if: github.repository == 'buildroot/buildroot'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v4
+        with:
+          pr-comment: |
+            Please do not submit a Pull Request via GitHub. Buildroot makes use of a
+            [mailing list](http://lists.buildroot.org/mailman/listinfo/buildroot) for patch submission and review.
+            See [submitting your own patches](http://buildroot.org/manual.html#submitting-patches) for more info.
+
+            Thanks for your help!
+          lock-pr: true
+          close-pr: true
+