cs8900: 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 cs8900 driver also changes slightly in that the hardware is not
consulted if the mac address in the env is sane.

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