Draft: [COMMON][C31][D23] Upgrade IMAGE_BOOT_FILES dtb inclusion: remove IMG_KERNEL_DEVICETREE

Until now IMAGE_BOOT_FILES included the device tree files using the variable IMG_KERNEL_DEVICETREE, removing the folder prefix from and duplicating the standard dtb definitions of KERNEL_DEVICETREE. To fix this we borrow an utility function from IMX BSP layer, copied as is from the conf/machine/include/utilities.inc file.