Instead of programming the hardware directly in the board
implementation, follow the flow documented in doc/README.enetaddr: set
the enet[0-9]*addr environment variable and let the driver program the
hardware.

This avoids duplicating the implementation as it already exists in the
driver (drivers/net/fec_mxc.c: fec_set_hwaddr).

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, i.e. FEC always has the index 2.

The FEC driver does *not* set the flag Set MAC Address on Transmit (bit
set_eth0_mac_address used to do but this is unnecessary as the Linux
networking stack fills in the MAC 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>