HI @Massimo,
when I execute the make command from ImageDev/DeviceTree/device-tree-overlays/overlays/ directory, many .dtbo file are created in the same directory.
No dtb file are created in ImageDev/linux/arch/arm/boot/dts/ directory.
You are mixing up device tree overlays with device trees here. As I already told you before, your module (Colibri iMX6ULL 256/512MB) does NOT support overlays. You should be working in the linux/arch/arm/boot/dts directory, and not in the device-tree-overlays/overlays/ directory. You should modify the necessary dts file in linux/arch/arm/boot/dts directory, execute the make command in the linux directory and this should generate the dtb files as I explained earlier.
The file to use to flash the device tree on Colibri, is a .dtb or a .dtbo ?
Since this module does not support overlays, you should be updating the ubi partition with the dtb files.