ubuntu_image/parser.py Gadget YAML Definition needs updating

[Emilio Lopez]

See original post on the snapcraft forum. https://forum.snapcraft.io/t/gadget-schema-for-ubuntu-image-parser-might-be-broken/14723

I am using `ubuntu-image` to make a ubuntu-core image. In the gadget.yaml definition, I added the `update` and `edition` definitions (link to documentation found in [The gadget snap](https://snapcraft.io/docs/gadget-snap) Snapcraft documentation.

The `ubuntu_image/parser.py`:181-236 does _not_ include the this in the schema. As a result calling `ubuntu_image` will fail because of that added key that is not in the schema. Is it expected to not have an update when making an image for the first time?

**Here is the Schema in parser.py**

    GadgetYAML = Schema({
        Optional('defaults'): {
            str: {
                str: object
            }
        },
        Optional('connections'): [Schema({
            Required('plug'): str,
            Optional('slot'): str,
            })
        ],
        Optional('device-tree-origin', default='gadget'): str,
        Optional('device-tree'): str,
        Optional('format'): YAMLFormat,
        Required('volumes'): {
            Match('^[a-zA-Z0-9][-a-zA-Z0-9]*$'): Schema({
                Optional('schema', default='gpt' if has_new_voluptuous()
                        else VolumeSchema.gpt):
                    Enumify(VolumeSchema),
                Optional('bootloader'): Enumify(
                    BootLoader, preprocessor=methodcaller('replace', '-', '')),
                Optional('id'): Coerce(Id),
                Required('structure'): [Schema({
                    Optional('name'): str,
                    Optional('offset'): Coerce(as_size),
                    Optional('offset-write'): Any(
                        Coerce(Size32bit), RelativeOffset),
                    Required('size'): Coerce(as_size),
                    Required('type'): Any('mbr', 'bare', Coerce(HybridId)),
                    Optional('role'): Enumify(
                        StructureRole,
                        preprocessor=methodcaller('replace', '-', '_')),
                    Optional('id'): Coerce(UUID),
                    Optional('filesystem', default='none' if has_new_voluptuous()
                            else FileSystemType.none):
                        Enumify(FileSystemType),
                    Optional('filesystem-label'): str,
                    Optional('content'): Any(
                        [Schema({
                            Required('source'): str,
                            Required('target'): str,
                            })
                        ], # noqa: E124
                        [Schema({
                            Required('image'): str,
                            Optional('offset'): Coerce(as_size),
                            Optional('offset-write'): Any(
                                Coerce(Size32bit), RelativeOffset),
                            Optional('size'): Coerce(as_size),
                            })
                        ],
                    )
                })]
            })
        }
    })

**Here is the YAML definition with the addition based off [pc-amd64-gadget:gadget.yaml](https://github.com/snapcore/pc-amd64-gadget/blob/18/gadget.yaml) on the 18 branch**

    volumes:
      pc:
        bootloader: grub
        structure:
          - name: mbr
            type: mbr
            size: 440
            content:
              - image: pc-boot.img
          - name: BIOS Boot
            type: DA,21686148-6449-6E6F-744E-656564454649
            size: 1M
            offset: 1M
            offset-write: mbr+92
            content:
              - image: pc-core.img
          - name: EFI System
            type: EF,C12A7328-F81F-11D2-BA4B-00A0C93EC93B
            filesystem: vfat
            filesystem-label: system-boot
            size: 50M
            content:
              - source: grubx64.efi
                target: EFI/boot/grubx64.efi
              - source: shim.efi.signed
                target: EFI/boot/bootx64.efi
              - source: mmx64.efi
                target: EFI/boot/mmx64.efi
              - source: grub.cfg
                target: EFI/ubuntu/grub.cfg
            update:
              - edition: 0

[Emilio Lopez (lopezem)]

Just updating tracking. PR was created: https://github.com/CanonicalLtd/ubuntu-image/pull/180