Allow downgrade prevention without the need to overwrite the received…

[kvaellning]

In certain scenarios, additional tests in the updated firmware are performed to determine the success of the update. Nevertheless, in those cases also the downgrade prevention is of interest.

This PR throws out the need activate the overwrite of the old image to avoid bricking of the device if a test run didn't work out.

The older image file will be deleted if the version is less than the current image version, to avoid more impact on the loader code (e.g. to deactivate the pending flag).