Mt7620a boot from TFTP requires a control_file


I'm trying to use TFTP to boot, in this case I'm unable to select any options in uboot (console doesn't take any input until image is loaded), but if the tftp server is setup it automatically connects to it. I have the bin file, but it also requires a "control_file" in the tftp dir. I can't figure out what the control_file is supposed to contain in order to boot from the bin?
If I leave the file blank, it just keep booting from flash. If I type anything else in it, it gets stuck as below.
Any ideas?

TFTP from server; our IP address is
Filename './control_file'.

 TIMEOUT_COUNT=3,Load address: 0x80200000
Loading: T T Got ARP REPLY, set server/gtwy eth addr (68:f7:28:36:57:8c)
Got it
Bytes transferred = 10 (a hex)
NetBootFileXferSize= 0000000a

 old,GDMA1_FWD_CFG 0xb0100600 = 20710007
board type : UC100

If I stringdump uboot.bin the only reference I see is:

downloadboot=tftp 0x80200000 ${tftpdir}/${bootname}
downloadcontrol=tftp 0x80200000 ./${controlfile}