Daniel,
You are using U-Boot from mainline for the mx6ul_14x14_evk board.
In this case, u-boot.imx will not be generated as this board supports SPL, so you will get: SPL and u-boot.img.
In order to flash the SD card:
$ sudo dd if=SPL of=/dev/mmcblk0 bs=1k seek=1; sync
(Note - the SD card node may vary, so adjust this as needed).
- Flash the u-boot.img image into the SD card:
sudo dd if=u-boot.img of=/dev/mmcblk0 bs=1k seek=69; sync