Allow setting GPT partition attributes in gadget.yaml

[Tobias Heider]

Some devices like for example Google Chromebooks use partition type specific GPT attributes to mark the boot partition. It would be nice if we had support for specifying flags in gadget.yaml to help ubuntu-image generate bootable images on such devices.

I imagine something like:

    structure:
      - name: Boot
        offset: 2048
        type: bare
        flags: 0x11f000000000000
        size: 65536
        content:
          - image: bootloader.bin

Alternatively there are also well-defined flags we might allow setting such as 'Hidden' or 'Read-only' which might make more sense as named constants, maybe something like:

        flags: 0x11f,hidden,ro

[Alfonso Sanchez-Beato (alfonsosanchezbeato)]

We have dicussed this in the foundations sync and it has been agreed that this requires a mini-spec with some more details.