内核下载：Wrong Image Format for bootm command

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开发板是SEED-DEC138，内核下载过程：硬件连接好后开启minicom，设置结束，开发板上电，hit any key to stop autoboot
然后设置U-Boot环境变量：
U-Boot > setenv serverip 192.168.40.128
U-Boot > setenv ipaddr 192.168.40.129
U-Boot > setenv bootargs mem=128M console=ttyS2,115200n8 root=/dev/nfs
U-Boot > nfsroot=192.168.40.128:/opt/dec138/nfs
U-Boot > ip=192.168.40.129:192.168.40.1:255.255.255.0
U-Boot > setenv bootcmd 'tftp 0xc0700000 uImage; bootm'
U-Boot > saveenv
保存后开发板reset，超级终端显示如下：U-Boot 2009.01 (Jan 06 2010 - 11:01:54)

I2C: ready
DRAM: 64 MB
NAND: NAND device: Manufacturer ID: 0xec, Chip ID: 0xdc (Samsung NAND 512MiB 3)
Bad block table found at page 262080, version 0x01
Bad block table found at page 262016, version 0x01
nand_read_bbt: Bad block at 0x11860000
nand_read_bbt: Bad block at 0x1b7a0000
512 MiB
In: serial
Out: serial
Err: serial
ARM Clock : 300000000 Hz
DDR Clock : 150000000 Hz
Net: More than one PHY detected.

Hit any key to stop autoboot: 0
Using device
TFTP from server 192.168.40.128; our IP address is 192.168.40.129
Filename 'uImage'.
Load address: 0xc0700000
Loading: T T T T T T T T T T T T T T T T T T T T
Retry count exceeded; starting again
然后就一直starting again 一直下载不了，手动停止后显示如下：
Loading: *
Abort
Wrong Image Format for bootm command
ERROR: can't get kernel image!
请教大牛们啊！
（之前测试过tftp是OK的）

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求助啊！