arm: 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.

Some warts are remaining and should be killed off (by moving the func to
the appropriate board init code):
	- davinci_eth_set_mac_addr
	- cs8900_get_enetaddr
	- smc_set_mac_addr

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