[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/SConscript

@@ -0,0 +1,14 @@
+# for network related component
+import os
+from building import *
+
+objs = []
+cwd = GetCurrentDir()
+list = os.listdir(cwd)
+
+for d in list:
+    path = os.path.join(cwd, d)
+    if os.path.isfile(os.path.join(path, 'SConscript')):
+        objs = objs + SConscript(os.path.join(d, 'SConscript'))
+
+Return('objs')