lldp: send the port name as the port description

A port named in the CLI or the web UI (port <n> name ...) now carries
that name in a Port Description TLV, so a neighbour sees "Uplink" next
to "Port 9". The tail of the frame is built per port, since the TLV
length differs from port to port; ports without a name send no TLV.

rtl837x_lldp.h

@@ -24,6 +24,7 @@
 #define LLDP_CHASSIS_ID_TLV_LENGTH 7
 #define LLDP_CHASSIS_ID_TLV_SUBTYPE 4
 
+#define LLDP_PORT_DESC_TLV_TYPE 4
 #define LLDP_SYSNAME_TLV_TYPE 5
 #define LLDP_SYSDESC_TLV_TYPE 6
 
@@ -47,5 +48,6 @@ void lldp_set_addresses(void) __banked;
 void lldp_set_rtl_wrapper(void) __banked;
 uint8_t lldp_sysname(__xdata uint8_t *p) __banked;
 uint8_t lldp_sysdesc(__xdata uint8_t *p) __banked;
+uint8_t lldp_portdesc(__xdata uint8_t *p, uint8_t port) __banked;
 
 #endif