All drivers should set the ESA by the same steps as the CS8900A, allowing
the user to override the ESA at the RedBoot command line if desired.

Note that RedBoot needs two options (see aaed2000 driver):eth0_esa and
eth0_esa_data, the former controlling whether to override the ESA or not,
the latter the actual ESA.

    // Find ESA - check possible sources in sequence and stop when
    // one provides the ESA:
    //   RedBoot option (via provide_esa)
    //   Compile-time configuration
    //   EEPROM
    //   <fail configuration of device>