U-boot is a very popular bootloader and you can learn more about it here: http://www.denx.de/wiki/U-Boot
Here is the trick to implement it:
setenv bootcmd 'dhcp; run cmd'
setenv cmd 'tftp LOADADDR PATH_TO_TFTPIMAGE'
In the above set of commands you will have to replace LOADADDR with the correct address in memory where you want to load the image. The PATH_TO_TFTPIMAGE is the path to the image on the tftpserver.
You will also need to set the serverip:
setenv serverip IPADDR
Finally, save the environment
savenvThese should work.
reset
Try the commands at your own risk. All info is provided for educational purposes only.