npe: 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 resulting code can also be simplified even further.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Stefan Roese <sr@denx.de>
CC: Ben Warren <biggerbadderben@gmail.com>