Issue title: Upgrade from standard OpenWRT possible?
 Issue body: Hey there,

first, thanks for your effort! Is it possible to use the sysupgrade image when my device is running a standard OpenWRT installation? Or is it possible to use the factory image when having a standard OpenWRT installation? Or do I have to go all the way back to using the stock image and starting over again with the factory image? Maybe this could go into the FAQ.

Thanks

Samy
 Comments: 
 Comment 0: Sysupgrade will do
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