It looks like that there is a problem of your terminal setting.
Check your terminal setting using the following pages:
http://www.armadeus.org/wiki/index.php?title=Connection_with_U-Boot_under_Windows
or
https://www.denx.de/wiki/DULG/SystemSetup
or
https://boundarydevices.com/just-getting-started/
Look at also attached file. Find the CONFIG_BOOTDELAY in this file. Perhaps the CONFIG_BOOTDELAY is set to 0 in your u-boot.
Have a great day,
Pavel Chubakov
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