Make sure that rootfs artifact payload has same checksum as sdimg partition

This is important for delta updates to work. Most likely this can be solved by having wic take the image as input instead of the rootfs folder

Acceptance criteria:

• Rootfs artifact payload must have same checksum as sdimg rootfs partition
  • If the partition is slightly larger than the artifact, then the only the size from the artifact should be checked
• Acceptance test which compares the checksums