Gangadhar,
Have you found the below guide for information on booting from USB:
There is some configuration that needs to be done for the BOOTP/DHCP server.
Can you tell if ethernet traffic is flowing at all over USB? Wireshark can be a great tool to help diagnose these kinds of issues. It's good to make sure you are seeing BOOTP messages and then see if they are requesting what you expect.
Keep us posted.