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.

Also rename load_sernum_ethaddr() to misc_init_r() so we don't need to
handle this board specially in common ARM code.

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