image: strip metadata from images when used in other artifacts

image: strip metadata from images when used in other artifacts

Image metadata and signature is of no use for images which are included
inside other artifacts (like an SD-card image). Strip them off before
using images in artifacts or stashing them for the ImageBuilder as the
contained signature breaks reproducibility.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 7a256d97d9ded84d1bfd531e775099774e7b6b06)