DFL can be activated at boot by sliding the power button and pressing the brightness buttton at the same time.

DTB was taken from https://github.com/avafinger/bananapi-zero-ubuntu-base-minimal/releases/download/v3.10/boot_focal.tar.gz and decompiled, then SPI interface was enabled according to https://github.com/avafinger/bananapi-zero-ubuntu-base-minimal/issues/49 .