Quick update: I managed to boot on a SD card containing an image from the beaglebone website (using the boot force method you gave)
I did not flash the MMC since I have my beaglebone black which is about 7 years old (2016) so when you mention an old version of u-boot, I think you are totally right.
I don’t have an usb-serial adapter for the moment but I should be able to get one in a few.
This is exactly what I have done to get the 4 solid led’s with my “custom debian” built using Debian: Getting Started with the BeagleBone Black.
What is the difference I should see with an updated version of u-boot ? Do we boot on the SD card without pushing on the boot button ?
Best regards,
Nicolas