[yocto] yocto_installer test script update

[Ke, Liping]

3. Correct
> the problem for qemu image download, now we forcing download images in
> a clean directory since wget will change the file ext if the file
> already there and which maybe corrupted 
> 
Hi, Jessica

Have question for this one.
Now the code will directly downloading, so the folders might contain image, image(1), image(2)...
But the untar will always untar the old image? Then why downloading new? If user's network is very slow?


Thanks& Regards,
criping