uboot的指令与支持的模块有关，下面以我这边的为例解释一下。
帮助
在串口下输入?或者help。
? - alias for 'help'
help - print command description/usage
base
base - print or set address offset
boot
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootk - boot kernel
bootm - boot application image from memory
bootz - boot Linux zImage image from memory
dhcp - boot image via network using DHCP/TFTP protocol
内存操作
cmp - memory compare
cp - memory copy
ddrtest - simple ddr capability check
md - memory display
mtest - simple RAM read/write test
mw - memory write (fill)
go - start application at address 'addr'
控制台
coninfo - print console devices and information
下载
downver - upgrade software downloaded from TFTP server
tftp - boot image via network using TFTP protocol
打印
echo - echo args to console
fdt - flattened device tree utility commands
gpio
gpiotest- gpiotest dir [num] [in/out]
gpiotest value [num] [1/0]
gpiotest gvalue [num]
imxtract- extract a part of a multi-image
itest - return true/false on integer compare
mii - MII utility commands
mtddebug- mtddebug operate
nand - NAND sub-system
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
setenv - set environment variables
sleep - delay execution for some time
version - print monitor, compiler and linker version
watchdog- watchdog reset && disable
xmodem - xmodem
