boot: bootutil: boot_record: Fix issue with saving image data

Fixes an issue with saving shared boot data when the number of
images is more than 1. This is caused becuase the function is
called once per image but the function itself saves common data
plus data for all images, so prevent saving when it has already
ran once

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>