Use a MAC address based on the SoC's UUID

We use the SoC's UUID to generate a fixed MAC for a particular
switch device. The MAC generated uses a Realtek prefix and then
is followed by 3 bytes genertaed from the first 3 bytes of the
UUID xored with the last byte in order to prevent being able
to deduce the UUID from the public MAC.

uip/uip.c

@@ -132,8 +132,6 @@ static __code const uip_ipaddr_t all_zeroes_addr =
   {0x0000,0x0000};
 #endif /* UIP_CONF_IPV6 */
 
-extern __code struct uip_eth_addr uip_ethaddr;
-
 #ifndef UIP_CONF_EXTERNAL_BUFFER
 u8_t uip_buf[UIP_BUFSIZE + 2];   /* The packet buffer that contains
 				    incoming packets. */