board: ge: bx50v3: program MAC address to I210

board: ge: bx50v3: program MAC address to I210

There are two I210s on the b850v3 and one on the b450v3 and b650v3.
One is connected to Marvell 88e6240 which is already programmed.

Follow the flow documented in doc/README.enetaddr: set the
enet[0-9]*addr environment variable and let the driver program the
hardware.

The mapping from the driver's index to the environment variable's name
is documented in README: Note for Redundant Ethernet Interfaces. It is
assumed that eth_devices for the controllers on the board are always
indexed in the same order.

The environment variables are removed after programming the hardware
because the variables seem to influence MAC addresses also after U-Boot.
Specifically the MAC address of FEC (MC interface) would be incorrectly
set: 'ethaddr', which maps to the first I210 chip and is set to I210's
default address read from the driver by eth_write_hwaddr in eth_legacy.c
because the variable is undefined (not set even by bx50v3.c), would
result in the eth0 interface's MAC address to be set to I210's default
address.

Signed-off-by: Hannu Lounento <hannu.lounento@ge.com>
Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>