Change rootfs_image_checksum over to use namespaced provides

Acceptance criteria:

• rootfs_image_checksum in mender-artifact must be renamed to rootfs-image.checksum to follow the new namespaced approach.
• There must be an option, --legacy-rootfs-image-checksum to use the old key name
• The change must be documented thoroughly in the changelog entry, including instructions for how to work around it when upgrading old devices.

Feature branch name: feature/partial_updates