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.

rtl837x_common.h

@@ -79,6 +79,7 @@ struct flash_region_t {
 };
 
 extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE+2];
+extern __xdata struct uip_eth_addr uip_ethaddr;
 
 // Headers for calls in the common code area (HOME/BANK0)
 void print_string(__code char *p);