sh_eth: get mac address from environment

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>