boards: move board_get_enetaddr() into board-specific init

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.

Rather than have the common ppc code have board-specific hooks, move the
board_get_enetaddr() function into the board-specific init functions.

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