Extend Mender-convert to keep original image name

The naming convention of the output image today is (not taking the .sdimg suffix in to account):

image_name="${device_type}-${artifact_name}"

This on raspberrypi results in to something like this,

1. Assuming that MENDER_ARTIFACT_NAME="release-1"
   raspberrypi-release-1.sdimg.gz

And there is no information extracted from the input disk image name. Should we extend this to do this as well?

That is,

1. Assuming that all suffixes have been stripped from disk_image name that was used as input
   image_name=${disk_image}

1. This is the actual file that will be written
   sdimg_path=deploy/${image_name}-mender.img