* [lwip-2.1.2] improve lwipopts.h

* 增加LWIP RTT侧版本定义

* 移动调整cc.h和lwipopts.h定义

* lwip2.1.2 2.0.3抽出lwipopts.h lwippools.h

* remove lwip1.4.1 lwipopts.h

* [update] modify the default config through v1.4.1

* delete the useless macro and modify some unclear comments.

* [lwipopts.h] 完善头文件相关宏以及临界区保护相关宏

* remove latest version label
Co-authored-by: Nliuxianliang <liuxianliang@rt-thread.com>

* [lwip] integrate lwip porting layer

* Lwip 通用文件梳理 (#2)

* [modify] extract the common files for different version about lwip.

* [update] the initialization logic of lwIP.

* 整理源码结构

* ethernetif 头文件 ""改为<>

* 整理代码

* add macro definition "RT_USING_DEVICE_IPC".

* [update] mandatory selected "RT_USING_DEVICE_IPC".

* [modify] compile error for lost "lwip/init.h"
Co-authored-by: Nxiangxistu <52819708+xiangxistu@users.noreply.github.com>
Co-authored-by: Nliuxianliang <liuxianliang@rt-thread.com>

* [net] adjust folder structure
- 将lwip相关的文件夹，移入lwip文件夹
- Kconfig 分散到at netdev lwip sal文件夹中，使用menuconfig代替menu

Signed-off-by: Nliuxianliang <liuxianliang@rt-thread.com>

Signed-off-by: NMurphyZhao <d2014zjt@163.com>