Per-partition .img files are no longer created for empty partitions [2.0]

[Łukasz Zemczak]

Consider the following gadget.yaml snippet from the structure list:

(...)
      - name: hyp
        size: 1048576
        content:
            - image: blobs/hyp.mbn
        type: E1A6A689-0C8D-4CC6-B4E8-55A4320FBD8A
      - name: sec
        size: 1048576
        type: 303E6AC3-AF15-4C54-9E9B-D9A8FBECF401
      - name: aboot
        size: 2097152
        content:
            - image: blobs/sd_appsboot.mbn
        type: 400FFDCD-22E0-47E7-9A23-F16ED9382388
(...)

The 'sec' partition is filesystem-less and has no 'content' stanza. With the new ubuntu-image 2.0 this partition will not have a partX.img created as our current code only does the dd for when a 'content:' is provided or a filesystem. This will cause such images to fail to build on the makedisk phase, as we'll have no partX.img to copy to the final disk file.

This causes UC18 dragonboard images to FTBFS.