For more information verify the U-Boot Manual.
You need the "lrzsz" package to add support on minicom to transfer over serial:
aptitude install lrzsz
Open Minicom and power-on the board. When the U-Boot prompt appears:
Type the command to transfer the u-boot.bin binary:
Then press the combination keys:
Then select the option:
A text mode "file explorer" will appear. Select the desired binary (u-boot.bin) pressing "Space" key.
The file transfer will start.
To execute the uploaded file just issue:
=> go 0x100000
Replace 0x100000 with your TEXT_BASE address.