LWIP_VLAN_PCP macro

LWIP_VLAN_PCP==1: Enable outgoing VLAN taggning of frames on a per-PCB basis for QoS purposes. With this feature enabled, each PCB has a new variable: "netif_hints.tci" (Tag Control Identifier). The TCI contains three fields: VID, CFI and PCP. - VID is the VLAN ID, which should be set to zero. - The "CFI" bit is used to enable or disable VLAN tags for the PCB. - PCP (Priority Code Point) is a 3 bit field used for Ethernet level QoS. See pcb_tci_*() functions to get/set/clear this.