* Resolve: Unable to boot from USB MSD - Seagate 5Tb HDD backup drive #139
* Increase USB MSD timeout from 10 to 20 seconds.
* Max retries now default to zero because the default BOOT_ORDER includes
restart (0xf). Therefore, each boot-mode is now tried once before moving
to the next mode. The retries mechanism is largely redudant now that
the loop/restart mode exists.
* If TFTP fails and network boot retries > 0 then wait 5 seconds before
retrying to avoid overloading a misconfigured TFTP server.
* Map undefined boot-modes in BOOT_ORDER to SD (0x1) instead of stopping.
* Add missing pieeprom-2020-05-28
