Hi, Guru
Agree with what Vladan said, it can only support serial down mode from USB, when internal boot mode failed or you select the serial download mode, USB will be detected as an HID device, you can use some tool(FSL should have such tool) to download a small program from PC to board, then the board will jump to run this program, normally we will download a uboot from USB port, then uboot will come out, and you can download kernel using the uboot,
i.MX25 can not boot directly from USB.
What it can do is initialize USB for communication and wait for instructions from PC on the other side. You could then write a small application on PC that downloads u-boot through USB to memory and runs it from there. u-boot could then load kernel and file system from net or from USB... It's not straightforward, but could be done, theoretically.