esp8266: Zero out fs_user_mount state on (soft) reset.

Otherwise device stays mounted on soft reset and leads to corruption
(since block device object is now gone).

esp8266/main.c

@@ -48,6 +48,7 @@ STATIC void mp_reset(void) {
     mp_init();
     mp_obj_list_init(mp_sys_path, 0);
     mp_obj_list_init(mp_sys_argv, 0);
+    memset(MP_STATE_PORT(fs_user_mount), 0, sizeof(MP_STATE_PORT(fs_user_mount)));
     MP_STATE_PORT(mp_kbd_exception) = mp_obj_new_exception(&mp_type_KeyboardInterrupt);
 #if MICROPY_MODULE_FROZEN
     pyexec_frozen_module("boot");