1. If eMMC is missing\faulty\completely erased - in these cases always need u-boot on SD card (run u-boot from a USB drive is possible, but I still know of only one manufacturer, which is implemented Khadas VIM on all the other TV boxes there is only a boot loader for the SD card).
2. All I know of u-boot in update mode have the option of running the script with the default name aml_autoscript.
3. Therefore, it is sufficient to write to the SD card standard u-boot and slip him the right script aml_autoscript (using as a basis a ready skriti s905_autoscript for the desired systems.
After recording of the image Armbian scripts aml_autoscript and s805_autoscript used for their own purposes. aml_autoscript is a script activate the multi-boot. S805_autoscript the script is a startup script system from external media.