I've looked at the image files for a number of devices that produce a factory.bin and I'm overwhelmed by the different modifications. I have the GPL'd source for my device, but it is missing *.dts files and a specific image mt7621.mk entry. I have UART output but u-boot seems to have the bootdelay set at zero.
Rather than hacking u-boot, I think the best course of action is to see if I can modify my sysupgrade.bin into a factory.bin. Can someone point to a methodical way to approach this? Specifically, I have a mt7621 based Trendnet device. I can post the UART output (1760 code lines split into several posts) if needed.