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_arp.h

@@ -55,8 +55,6 @@
 #include "uip.h"
 #include "../rtl837x_common.h"
 
-extern __code struct uip_eth_addr uip_ethaddr;
-
 /**
  * The Ethernet header.
  */