boards: get mac address from environment

The boards that get converted here to use the environment for the mac
address rather than global data:
	debris
	mgcoge
	mgsuvd
	muas3001
	netstal
	pn62
	sixnet
	vcma9
	xilinx (the ones that use xilinx_enet)

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Ben Warren <biggerbadderben@gmail.com>
CC: Sangmoon Kim <dogoil@etinsys.com>
CC: Heiko Schocher <hs@denx.de>
CC: David Mueller <d.mueller@elsoft.ch>
CC: Niklaus Giger <niklaus.giger@netstal.com>
CC: Wolfgang Grandegger <wg@denx.de>
CC: Dave Ellis <DGE@sixnetio.com>
CC: Ricardo Ribalda <ricardo.ribalda@uam.es>