utils: checkstyle.py: Warn when no valid Signed-off-by line is found

All commits to libcamera must include a Signed-off-by line, and that
rule is enforced through git hooks and CI. This however doesn't prevent
patches from being submitted without an SoB tag, as noticed multiple
times in the past. Extend the checkstyle.py trailer checker to issue a
warning when the SoB line is missing to try and improve the situation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/checkstyle.py

@@ -493,6 +493,8 @@ class TrailersChecker(CommitChecker):
     def check(cls, commit, top_level):
         issues = []
 
+        sob_found = False
+
         for trailer in commit.trailers:
             match = TrailersChecker.trailer_regex.fullmatch(trailer)
             if not match:
@@ -515,6 +517,12 @@ class TrailersChecker(CommitChecker):
                 issues.append(CommitIssue(f"Malformed value '{value}' for commit trailer '{key}'"))
                 continue
 
+            if key == 'Signed-off-by':
+                sob_found = True
+
+        if not sob_found:
+            issues.append(CommitIssue(f"No valid 'Signed-off-by' trailer found, see Documentation/contributing.rst"))
+
         return issues