cpu/: 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 cpus that get converted here:
	at91rm9200
	mpc512x
	mpc5xxx
	mpc8260
	mpc8xx
	ppc4xx

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