[Booting from external and removable storage] not applicable for u-boot 2019.07 and newer

Both

https://doc.turris.cz/doc/en/howto/omnia_booting_from_external_storage#booting_from_ssd
https://doc.turris.cz/doc/en/howto/omnia_booting_from_external_storage#booting_from_usb

cite btrload which though is a legacy command and produces

Unknown command 'btrload' - try 'help'