repart: add --image= switch

repart: add --image= switch

This is similar to the --image= switch in the other tools, like
systemd-sysusers or systemd-tmpfiles, i.e. it apply the configuration
from the image to the image.

This is particularly useful for downloading minimized GPT image, and
then extending it to the desired size via:

   # systemd-repart --image=foo.image --size=5G