SDK tarballs have a .DIGESTS file but it is created by catalyst instead
of the upload_image function. In order to support plain GPG signing but
not avoid re-generating .DIGESTS we need to move that code out of
upload_image to a new function. upload_files shouldn't do it itself
because it is also used for portage binary packages which shouldn't be
signed (there is no point, nothing would verify the signatures).
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