Consider u-boot as a BIOS on a PC and everything falls into place, you do not need to interfere with its operation. No one in the field of development of Debian|Ubuntu and other distributions for PC does not develop BIOS for PC, this is a direct task of the manufacturer, who knows all the features of their hardware. Similarly to u-boot for TV boxes, let the manufacturer deal with it, our task is to add to it the minimum means of starting any system absolutely and, if possible, installing the system in internal memory (eMMC). The main u-boot will never be able to work fully on all TV boxes.
Stupid advice, kill your device and then hope that you do not have to restore it in complex steps (which can not be performed, even experienced users, not to mention ordinary users). It is not difficult to erase the u-boot, but if you try to write a wrong (not working u-boot) to eMMC, you will get a hard-to-recover brick. So my advice is never to listen to such idiotic advice and be very careful with u-boot.