scripts/checklayer: check for SECURITY.md

Add a check for a SECURITY.md file (or similar) to yocto-check-layer, as
knowing where to report security issues is important.

(From OE-Core rev: c7cb22ac4ceed60f88452e83c907a4c4f33660e4)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/checklayer/__init__.py

@@ -452,3 +452,15 @@ def compare_signatures(old_sigs, curr_sigs):
             msg.extend(['      ' + line for line in output.splitlines()])
             msg.append('')
     return '\n'.join(msg)
+
+
+def get_git_toplevel(directory):
+    """
+    Try and find the top of the git repository that directory might be in.
+    Returns the top-level directory, or None.
+    """
+    cmd = ["git", "-C", directory, "rev-parse", "--show-toplevel"]
+    try:
+        return subprocess.check_output(cmd, text=True).strip()
+    except:
+        return None