To bring up the board you need to load u-boot prior to Linux.
I mean that boot ROM code of the processor loads u-boot from some media and then u-boot loads Linux kernel.
If you are using some stadnard boadd you can use u-boot from its binary demo files.
For example here you can get u-boot-imx6ulevk_sd.imx file for i.MX6UltraLite Evaluation Kit board:
https://www.nxp.com/support/developer-resources/software-development-tools/i.mx-developer-resources/...
When the board is running u-boot you can see some diagnostic output in terminal.
And finally you can load Linux kernel using u-boot.
Have a great day,
Victor
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