After the first half of the command, you should already have a uart version of u-boot running in your RAM, but:
1) you need to make sure you are downloading the correct version, the uart version of u-boot, I guess you have but it does not hurt to double check.
2) this running u-boot only exits in the RAM, means if you reset it, it's gone, you need to start the first half again to restore it in u-boot.
After first half is done, hit any key on your keyboard you should see the u-boot console on your terminal program.
More thoughts about your trouble shooting:
1) try to verify your minicom setting is correct, this would include a) the serial device is set to /dev/ttyUSB0 b) 57600-8-N-1. And better
2) check your hardware cable, the TX should works fine since first half get passed, but how about RX? A simple way is to short the PIN 2 and 3 of your DB9 connector, type in the minicom, you should get you typed displayed immediately.
3) If possible, have another board to verify both your minicom setting and cables are working fine.
