fdt: rework fdt_fixup_ethernet() to use env instead of bd_t

Move to using the environment variables 'ethaddr', 'eth1addr', etc..
instead of bd->bi_enetaddr, bi_enet1addr, etc.

This makes the code a bit more flexible to the number of ethernet
interfaces.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>