If you are using “minicom” to communicate with U-Boot and want to transfer Linux kernel image over serial you should first install lrzsz package:
$ sudo apt-get install lrzsz
Then on minicom issue this command on U-Boot prompt:
=> loady ## Ready for binary (ymodem) download to 0xA0800000 at 115200 bps... C
Now press Ctrl + A, then release keyboard and press S. It will show a text window to select what protocol to use, then select ymodem. Now select a file to upload (press Space) and press Enter to confirm.