Hello,
To enter on fastboot mode you need:
1. Turn on the board and stop boot flow pressing any key to enter in U-boot.
2. In U-boot enter next command:
fastboot 0
Use the next command to send the file with UUU:
uuu -v -b fat_write <file> mmc 0:1 <file>
Example:
uuu -v -b fat_write hello_world.elf mmc 0:1 hello_world.elf
Best regards.