When u-boot is loaded with imx_usb utility, it will start immediately and present s short countdown 2-1-0. In this time frame you need to press a key in a your console terminal. So use both of your hands,start the download with one hand and use the other hand to press a key.
To simplify this I have the console terminal on a seperate Windows PC and the imx_usb utility is on my Linux machine. Minicom is a pain in itself anyway :-)
But you can also set a higher value for the wait period in the u-boot environment variables, then it's easier to handle. So start the board normally and interrupt the loading process with a key press, then set the new value in u-boot.
