Thank you Kamil_S for the file.
If you have the opportunity to boot also using the resulting u-boot from this build and record a full boot log (u-boot + kernel) together with a login and ifconfig right after it gets IP would be very helpful.
Please write u-boot to SD Card using this command:
sudo dd if=./u-boot-rt/u-boot.bin of=/dev/sdX bs=1k seek=40
where X is your sd card device letter or the correspondent mmcblkZ if you use internal sd card reader/writer where Z is your device number.
if you have an embedded sd card reader (notebook/laptop) you get something like this:
[63376.329036] mmc0: new SDHC card at address 1234
[63376.368234] mmcblk0: mmc0:1234 SA04G 3.67 GiB
[63376.368372] mmcblk0: p1 p2
in this example sd card is /dev/mmcblk0 so the format is something like /dev/mmcblkX where X is a number