target: Always read ubootenv from active slot
target: Always read ubootenv from active slot

This way, we'll never overwrite the factory ubootenv image, but
if for some reason it becomes the active partition, we can at least
get boot info from that.

SilverLiningSystems/cxmanage-test,SilverLiningSystems/cxmanage-test,Cynerva/cxmanage,SLS-Dev/cxmanage,Cynerva/cxmanage,SilverLiningSystems/cxmanage-test2,SilverLiningSystems/cxmanage-test2,SLS-Dev/cxmanage