ETHARP_SUPPORT_VLAN macro

ETHARP_SUPPORT_VLAN==1: support receiving and sending ethernet packets with VLAN header. See the description of LWIP_HOOK_VLAN_CHECK and LWIP_HOOK_VLAN_SET hooks to check/set VLAN headers. Additionally, you can define ETHARP_VLAN_CHECK to an u16_t VLAN ID to check. If ETHARP_VLAN_CHECK is defined, only VLAN-traffic for this VLAN is accepted. If ETHARP_VLAN_CHECK is not defined, all traffic is accepted. Alternatively, define a function/define ETHARP_VLAN_CHECK_FN(eth_hdr, vlan) that returns 1 to accept a packet or 0 to drop a packet.