For more information verify the U-Boot User Manual.
First all configure your board IP, your host IP and the gateway IP:
=> setenv ipaddr 10.29.244.91
=> setenv serverip 10.29.244.27
=> setenv gatewayip 10.29.244.27
Save this configuration to flash:
Download the file to board RAM:
=> tftp 0x80000000 zImage
400000 is the memory position where the file will be placed;
zImage is the file that will be downloaded from TFTP server.