sandbox: Move some mac addresses to device tree

This prevents some conflicts when running sandbox with -D, since the
"rom" mac address will be random and won't match the environment. We
still need to keep addresses for eth1 and eth6 in the environment,
because dm_test_eth_rotate expects to be able to disable them by
removing their envaddr variables. This can likely be fixed in a future
series by adding a function to cause sandbox eth_opts callback for a
particular mac to fail immediately.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>