The environment is the canonical storage location of the mac address, so
we're killing off the global data location and moving everything to
querying the env directly.

The sh_eth driver can also be simplified a bit by using enetaddr member of
the eth_device structure.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
CC: Carlos Munoz <carlos@kenati.com>
CC: Ben Warren <biggerbadderben@gmail.com>