I think you have it backwards. Your Ubuntu PC is the tftp server. If you have the SDK, look in the docs folder for sitara-linuxsdk-sdg-xx.xx.00.00.pdf. In this file, do a search for tftp to see how to configure tftp.
If you don't have the SDK, see:
http://processors.wiki.ti.com/index.php?title=Sitara_Linux_Software_Developer%E2%80%99s_Guide&oldid=171586
