各位专家好，请问下我用sfh_OMAP-L138.exe -flash -targetType AM1808 -flashType NAND
ubl_OMAPL138_NAND.bin u-boot.bin -P COM1
后，引导uboot失败，是什么原因？提示如下：
AM1808 initialization passed!
Booting TI User Boot Loader
UBL Version: 1.65
Device OPP (300MHz, 1.2V)
UBL Flashtype: NAND
Starting NAND Copy...
No valid boot image found!
NAND Boot failed. Aborting...
