How to update U-Boot on pico-imx7d board
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Required software on the host PC:

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

Build U-Boot for pico:

$ make mrproper
$ make pico-imx7d_defconfig
$ make

This generates the U-Boot binary called u-boot.imx.

Put pico board in USB download mode (refer to the PICO-iMX7D Quick Start Guide
page 3)

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 starts and its messages appear in the console program.

Use the default environment variables:

=> env default -f -a
=> saveenv

Run the DFU agent so we can flash the new images using dfu-util tool:

=> dfu 0 mmc 0

Flash SPL into the eMMC:

$ sudo dfu-util -D SPL -a spl

Flash u-boot.img into the eMMC:

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

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.