net: Add VLAN device support

Inspired by Linux's way, we also create VLAN devices for managing VLAN,
which will become interfaces like `eth0.58`.

QinQ is also supported, we can create VLAN devices above another VLAN
devices, like `eth0.100.101` (or even `eth0.1.2.3.4`, also supported on
Linux).

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

drivers/net/CMakeLists.txt

@@ -65,6 +65,10 @@ if(CONFIG_NET)
     list(APPEND SRCS tun.c)
   endif()
 
+  if(CONFIG_NET_VLAN)
+    list(APPEND SRCS vlan.c)
+  endif()
+
   if(CONFIG_NET_FTMAC100)
     list(APPEND SRCS ftmac100.c)
   endif()