[net] adjust folder structure (#5587)

* [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

components/net/lwip-nat/SConscript

@@ -0,0 +1,10 @@
+from building import *
+
+cwd = GetCurrentDir()
+src = Glob('*.c')
+
+CPPPATH = [cwd]
+
+group = DefineGroup('lwIP', src, depend = ['RT_USING_LWIP', 'LWIP_USING_NAT'], CPPPATH = CPPPATH)
+
+Return('group')