B20 Reinstall uImage

1. Update uImage on uboot mode 
 a. ready tftp Server,include the uImage directory

 b. get into uboot mode

step 1:

#set serverip
#set ipaddr
must  show alive 

you must disable the firewall of your tftp server computer,otherwise tftp will be blocked
step 2:
#sf erase 40000 7c0000;tftp 80200000 iSpeaker-ver-1.0.11.bin;

notice:the ${filesize} is  output result last step,like 55bd1c 

#sf write 80200000 40000 ${filesize};reset 

step3:(log in uboot mode again)

#set imagename iSpeaker-ver-1.0.11;save;reset