This flag uses InkVT (from NiLuJe's fork: https://github.com/NiLuJe/inkvt , thanks to @llandsmeer too) to display the entire output of the rcS on the eInk screen when it usually is on the serial console. It is available for both standard and rooted kernels, and can be enabled by creating a file named "DISPLAY_DEBUG" and with "true" as its contents. Likewise, it can be disabled by writing "false" into it.
It can be used to provide debug information or diagnose problems more easily without needing to connect to a serial console.

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 .

DFL (Direct Firmware Loader) mode is a special boot environment where InkBox OS exports the entire MMC over USB storage. The user has 3 seconds from ~ when the green light starts blinking (kernel is loaded) to press both Home and Power buttons to enter DFL mode. If only one of these two or none at all is activated, the boot process continues as normal.

Causes OpenRC in rootfs chroot to behave strangely...

c.f. https://github.com/kobolabs/Kobo-Reader/issues/105

Little blue light staying put and nothing launches, complete system hang.
Why? It is a pure mystery, my friend.