freebsd-universe: Fix type of explicit_subdirs_only

Iteration over it is governed by a truthy check so the use of False
rather than [] was harmless, but some typecheckers don't have enough
flow-sensitive analysis to know that, and this abuse of types was not
intended, so just make it more sensible.

Fixes: 085f75f ("cheribsd: Overhaul freebsd-univere and add cheribsd-universe")

Author: jrtc27

pycheribuild/projects/cross/cheribsd.py

@@ -858,7 +858,7 @@ def setup_config_options(
             cls.build_lib32 = False
 
         if cls.universe_target:
-            cls.explicit_subdirs_only = False
+            cls.explicit_subdirs_only = []
             return
 
         cls.explicit_subdirs_only = cls.add_list_option(