As part of the fix, make the IP address private to the ipv4.c

sockitowm:temp28: modify reading temperature function so that it could be used in WRPC, no 1s delays, no floating point

Update Etherbone config space with correct IP address.

wrc_main starts bootp whenever link goes up
Rever Arria2 sys clock to 125MHz -- we had packet loss at 62.5MHz
Added IP training support to the shell
Have the shell reset the packet filter when the MAC is changed

Tidy up printing at startup

Add more optional debug statements.

Read the MAC address either from EEPROM or sensor ID (preferring EEPROM).

This commit can be omitted when merging back to master, but not all the following commits.

Initialize IPv4 after posix network startup.

Makefile: added Etherbone enable/disable switch, fixed --gc-sections linker arg (saves 6 kB of code)

Hook a new socket to tap ARP messages
Process ARP messages into replies