py: Implement a module system for external, user C modules.

This system makes it a lot easier to include external libraries as static,
native modules in MicroPython.  Simply pass USER_C_MODULES (like
FROZEN_MPY_DIR) as a make parameter.

py/objmodule.c

@@ -31,8 +31,6 @@
 #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);