Hi @drewmoseley.
After test some images today I finally got an image with just U-BOOT running in var-dart-imx8mm with BSP 5.4.3 from Variscite (they do not support yet the new BSP for this board).
I just had to add this line to u-boot-variscite.bbappend to the board boot up succesfully to kernel.
KERNEL_DEVICETREE = “freescale/imx8mm-var-dart.dtb”