The latest U-Boot for i.MX6ULL can be found in the NXP's official website. The U-Boot is a universal bootloader with support for many CPU architectures, including ARM (ARM7, ARM9, StrongARM, Xscale), which is used in the i.MX6ULL.
For the i.MX6ULL, you can download the U-Boot source code from the official NXP website. The U-Boot source code comes with a GPL license, which means you can modify and distribute it under the same license.
Once you have downloaded the U-Boot source code, you can set up the U-Boot and toolkit, make and program the image according to the instructions provided in the U-Boot quick start guide. The guide provides a simple user guide on how to use U-Boot on the i.MX6ULL MCU.
