"It spits put some garbage" I know this sounds trivial but recheck that you are using the right serial UART and baud rate.

"but I'm not seeing any output on serial port when it's supposed to be writing stuff out" Also check that the TTL level to RS-232 logic level transceiver is enabled (the MAX232 or equivalent).

Also, look at the uart buffer in memory and make sure u-boot gets that far and sends stuff to the UART.

-JM