py/builtin: Clean up and simplify import_stat and builtin_open config.

The following changes are made:

- If MICROPY_VFS is enabled then mp_vfs_import_stat and mp_vfs_open are
  automatically used for mp_import_stat and mp_builtin_open respectively.

- If MICROPY_PY_IO is enabled then "open" is automatically included in the
  set of builtins, and points to mp_builtin_open_obj.

This helps to clean up and simplify the most common port configuration.

Signed-off-by: Damien George <damien@micropython.org>

ports/minimal/main.c

@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "py/builtin.h"
 #include "py/compile.h"
 #include "py/runtime.h"
 #include "py/repl.h"