Make OS_SysInfo(4) generated MAC a unicast address
Recent issues on some Pi3 kit in the EtherUSB module showed that the return of OS_SysInfo(4) included the multicast bit (r1 bit 24) set. This is not wise and causes problems with modern internet stacks. If the address is read from the network card, HAL, or generated from the Acorn ID chip the return is untouched. The last ditch is to generate one from the machine random generated ID if it has one. Bit 24 is now clear and bit 25 is now set in the address thus generated. Bit 24 is unicast/multicast. Bit 25 set indicated a locally allocated address rather than a globally allocated one.