How to Update U-Boot on Pico-imx6ul board
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Required software on the host PC:

- imx_usb_loader: https://github.com/boundarydevices/imx_usb_loader

- dfu-util: http://dfu-util.sourceforge.net/releases/

Build U-Boot for Pico:

$ make mrproper
$ make pico-imx6ul_defconfig
$ make

This will generate the U-Boot binary called u-boot.imx.

Put pico board in USB download mode (refer to the document
http://www.wandboard.org/images/hobbit/hobbitboard-imx6ul-reva1.pdf page 15)

Connect a USB to serial adapter between the host PC and pico

Connect a USB cable between the OTG pico port and the host PC

Open a terminal program such as minicom

Copy u-boot.imx to the imx_usb_loader folder.

Load u-boot.imx via USB:

$ sudo ./imx_usb u-boot.imx

Then U-Boot should start and its messages will appear in the console program.

Use the default environment variables:

=> env default -f -a
=> saveenv

Run the DFU command:
=> dfu 0 mmc 0

Transfer u-boot.imx that will be flashed into the eMMC:

$ sudo dfu-util -D u-boot.imx -a boot

Then on the U-Boot prompt the following message should be seen after a
successful upgrade:

#DOWNLOAD ... OK
Ctrl+C to exit ...

Remove power from the pico board.

Put pico board into normal boot mode

Power up the board and the new updated U-Boot should boot from eMMC.