如何切换开发板的用户啊？

风潇潇姿姣姣

MYSAMA5EK200> tftp 20008000 text           
gmac0: PHY present at 5
gmac0:5 is connected to gmac0.  Reconnecting to gmac0
gmac0: Starting autonegotiation...
gmac0: Autonegotiation complete
gmac0: link up, 100Mbps full-duplex (lpa: 0x4de1)
Using gmac0 device
TFTP from server 192.168.18.10; our IP address is 192.168.18.81
Filename 'text'.
Load address: 0x20008000
Loading: #
         1000 Bytes/s
done
Bytes transferred = 17 (11 hex)
MYSAMA5EK200> md 20008000 text
MYSAMA5EK200> md 20008000
MYSAMA5EK200> md 20008000
MYSAMA5EK200> su
Unknown command 'su' - try 'help'
MYSAMA5EK200> root
Unknown command 'root' - try 'help'
MYSAMA5EK200> whoarm
Unknown command 'whoarm' - try 'help'
MYSAMA5EK200>

软件01

您好，这是U-BOOT，怎么切换，需要进入系统