By default our latest Embedded Linux BSPs deploy u-boot-nand.imx now in a folder called colibri_vf to allow for multiple module’s files to co-exist. Please copy the file also to the root folder of the first partition on that SD card. Alternatively depending on the EBoot version you may be able to specify the full path (e.g. flashloader colibri_vf\u-boot-nand.imx). I leave it to @luka.tx to comment on which version would support this.