buildroot/board/boundarydevices/common/genimage.cfg

image sdcard.img {
	hdimage {
	}

	partition u-boot {
		in-partition-table = "no"
		image = "u-boot.imx"
		offset = 1024
	}

	partition rootfs {
		partition-type = 0x83
		image = "rootfs.ext4"
		size = 512M
	}
}