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），我按照运行linux的步骤进行操作来自here的am-1808董事会：
我找不到任何可以解释从usb启动linux的官方参考/文档，我可以将文件系统+内核放在usb中，然后告诉uboot查找usb以查找uImage来自usb usb而不是SD卡。如果是，那么请你解释一下如何？
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代替sd-card来放置文件系统+ uImage，因为我必须使用SD卡引脚（mmc0）用于其他目的。
更新
This解释了从usb启动linux，但问题是u-boot不能识别＆＃39; usb＆＃39;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'
＆＃39; usb＆＃39;命令也不会出现在＆＃39; help＆＃39;显示的可用命令列表中。命令。
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
我知道如何指定＆＃39; usb＆＃39; u-boot中的命令？我需要将uboot升级到更新的版本吗？
问候
乌萨马