Added random_mac configuration option to ethernet.

    * peripheral/eth.c (eth_random_mac): Added configuration option to
    control the MAC address generation from the simulator.
    * doc/or1ksim.texi: Added information on random_mac
    * configuration.
    * doc/or1ksim.info: Regenerated.
    * doc/version.info: Regenerated.
    * sim.cfg: Updated sample configuration to list random_mac.