I have been able to boot a kernel compiled on ubuntu 9.04 using tftp and u-boot on the sd card that came with the qsb.
I followed the below instructions and built a zImage kernel and u-boot image on an ubuntu 12.04 machine. u-boot runs but it complains about uEnv.txt and the zImage. Should these instructions work?
i.MX53 Quick Start - Linux on ARM - eewiki
EDIT: I converted this zImage to a uImage with the mkimage tool and it booted ok, so the zImage is OK. However my question still remains.
It is better to follow the instructions from the User Guide, give it a try.
Leo