github: Setup protection for master branch.

* We do not have "Settings" tab in the Apache's owned repository,
  thus we need to update .asf.yaml file with repository settings.
* No direct push to master branch is possible.
* Require status checks to pass before merge.
* Setup reviews parameters.
* Require signatures.
* Require conversation resolution.
* Require linear history.

Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>

.asf.yaml

@@ -1,10 +1,9 @@
+# Reference: https://github.com/apache/infrastructure-asfyaml.
 github:
   description: "Apache NuttX is a mature, real-time embedded operating system (RTOS)"
   homepage: https://nuttx.apache.org/
   features:
-    # Enable issues management
     issues: true
-    # Enable project for project management boards
     projects: true
   labels:
     - nuttx
@@ -14,9 +13,21 @@ github:
     - mcu
     - microcontroller
   enabled_merge_buttons:
-    # disable squash button:
     squash: false
-    # disable merge button:
     merge: false
-    # enable rebase button:
     rebase: true
+  protected_branches:
+    master:
+      required_status_checks:
+        strict: true
+        contexts:
+          - Check
+          - Lint
+          - Build
+      required_pull_request_reviews:
+        dismiss_stale_reviews: true
+        require_last_push_approval: true
+        required_approving_review_count: 2
+      required_linear_history: true
+      required_signatures: true
+      required_conversation_resolution: true