Fix preflight complex extend handling (#21191)

buildroot/share/PlatformIO/scripts/preflight-checks.py

@@ -1,6 +1,6 @@
 #
 # preflight-checks.py
-# Script to check for common issues prior to compiling
+# Check for common issues prior to compiling
 #
 import os
 import re
@@ -25,6 +25,9 @@ def check_envs(build_env, base_envs, config):
         return True
     ext = config.get(build_env, 'extends', default=None)
     if ext:
+        if isinstance(ext, str):
+            return check_envs(ext, base_envs, config)
+        elif isinstance(ext, list):
             for ext_env in ext:
                 if check_envs(ext_env, base_envs, config):
                     return True