cmc_pu2: 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.

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>