esp8266/modesp: Add esf_free_bufs() debugging function.

Return number of free inernal WiFi buffers.

esp8266/modesp.c

@@ -33,6 +33,7 @@
 #include "py/obj.h"
 #include "py/gc.h"
 #include "py/runtime.h"
+#include "py/mphal.h"
 #include "netutils.h"
 #include "queue.h"
 #include "ets_sys.h"
@@ -645,6 +646,11 @@ STATIC mp_obj_t esp_meminfo() {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(esp_meminfo_obj, esp_meminfo);
 
+STATIC mp_obj_t esp_esf_free_bufs(mp_obj_t idx_in) {
+    return MP_OBJ_NEW_SMALL_INT(ets_esf_free_bufs(mp_obj_get_int(idx_in)));
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(esp_esf_free_bufs_obj, esp_esf_free_bufs);
+
 STATIC const mp_map_elem_t esp_module_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_esp) },
 
@@ -664,6 +670,7 @@ STATIC const mp_map_elem_t esp_module_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR_freemem), (mp_obj_t)&esp_freemem_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_meminfo), (mp_obj_t)&esp_meminfo_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_info), (mp_obj_t)&pyb_info_obj }, // TODO delete/rename/move elsewhere
+    { MP_OBJ_NEW_QSTR(MP_QSTR_esf_free_bufs), (mp_obj_t)&esp_esf_free_bufs_obj },
 
 #if MODESP_INCLUDE_CONSTANTS
     { MP_OBJ_NEW_QSTR(MP_QSTR_SLEEP_NONE),