我有Am-1808板（evm zoom kit），现在我在 spi flash 中有 ubl 和 uboot，在 SD 卡中有文件系统和内核（uImage），我按照在 am-1808 板上运行 linux 的步骤从这里：
我没有找到任何可以解释从 USB 启动 linux 的官方参考/文档，是否有可能我可以将文件系统 + 内核放在 USB 中，然后告诉 uboot 查找 USB 以从 USB 而不是 sd 查找 uImage-卡片。如果是，那么你能解释一下如何吗？
从 sd 卡中查找 uImage 的 u-boot 代码部分是：
#define CONFIG_BOOTARGS "mem=32M console=ttyS2,115200n8 root=/dev/mmcblk0p2 rw rootwait ip=off"
#define CONFIG_BOOTCOMMAND "if mmc rescan 0; then if fatload mmc 0 0xc0600000 boot.scr; then source 0xc0600000; else fatload mmc 0 0xc0700000 uImage; bootm c0700000; fi; else sf probe 0; sf read 0xc0700000 0x80000 0x220000; bootm 0xc0700000; fi"
我需要使用 usb 而不是 sd 卡来放置文件系统 + uImage，因为我必须将 sd 卡引脚 (mmc0) 用于其他目的。
更新：
这解释了从 USB 启动 linux，但问题是 u-boot 无法识别“usb”命令，当我输入：usb reset 时，它给了我这个错误：
U-Boot > usb reset
Unknown command 'usb' - try 'help'
“usb”命令也不会出现在“help”命令显示的可用命令列表中。
U-Boot > help
? - alias for 'help'
askenv - get environment variables from stdin
base - print or set address offset
bdinfo - print Board Info structure
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootm - boot application image from memory
bootp - boot image via network using BOOTP/TFTP protocol
cmp - memory compare
coninfo - print console devices and information
cp - memory copy
crc32 - checksum calculation
dhcp - boot image via network using DHCP/TFTP protocol
echo - echo args to console
editenv - edit environment variable
env - environment handling commands
exit - exit script
ext2load- load binary file from a Ext2 filesystem
ext2ls - list files in a directory (default /)
false - do nothing, unsuccessfully
fatinfo - print information about filesystem
fatload - load binary file from a dos filesystem
fatls - list files in a directory (default /)
go - start application at address 'addr'
help - print command description/usage
iminfo - print header information for application image
imxtract- extract a part of a multi-image
itest - return true/false on integer compare
loadb - load binary file over serial line (kermit mode)
loads - load S-Record file over serial line
loady - load binary file over serial line (ymodem mode)
loop - infinite loop on address range
md - memory display
mdc - memory display cyclic
mii - MII utility commands
mm - memory modify (auto-incrementing address)
mmc - MMC sub system
mmcinfo - display MMC info
mtest - simple RAM read/write test
mw - memory write (fill)
mwc - memory write cyclic
nfs - boot image via network using NFS protocol
nm - memory modify (constant address)
ping - send ICMP ECHO_REQUEST to network host
printenv- print environment variables
reset - Perform RESET of the CPU
run - run commands in an environment variable
saveenv - save environment variables to persistent storage
saves - save S-Record file over serial line
setenv - set environment variables
sf - SPI flash sub-system
showvar - print local hushshell variables
sleep - delay execution for some time
source - run script from memory
sspi - SPI utility command
test - minimal test like /bin/sh
tftpboot- boot image via network using TFTP protocol
true - do nothing, successfully
version - print monitor version
知道如何在 u-boot 中指定“usb”命令吗？我需要将uboot升级到更新版本吗？
问候
乌萨马