uboot命令说明:
? - alias for help
bootm - boot
ddr_greenbox - ddr_greenbox test
ddr_ssn - ddr_ssn test
go - start application at address 'addr'
help - print command help
md - memory display
mdio - mdio sub-system
menu - menu
mw - memory write (fill)
nand - NAND sub-system
ping - send ICMP ECHO_REQUEST to network host
printenv - print env
reset - Perform RESET of the CPU
run - run command that saved in env
saveenv - save env
setenv - set env
sfc - sfc sub-system
sfc_nand - sfc_nand sub-system
tftp - tftp [loadAddress] filename
upg - upg alias