py: Update and rework build system for including external C modules.

How to use this feature is documented in docs/develop/cmodules.rst.

py/objmodule.c

@@ -31,6 +31,8 @@
 #include "py/runtime.h"
 #include "py/builtin.h"
 
+#include "genhdr/moduledefs.h"
+
 STATIC void module_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_module_t *self = MP_OBJ_TO_PTR(self_in);