ppc: mark global bi_enet*addr as legacy

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.

In the ppc case, these things are part of the legacy ABI, so keep them
around but mark them as legacy so no new code will touch them.

Also stop calling load_sernum_ethaddr() since all boards now implement
this as a stub.

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