bootc: add support for /usr/lib/bootc-image-builder/disk.yaml

[mvo5]

Once we do this we need to consider this part of the YAML
an API. This should be fine as its pretty stable already
(and we need a test that has all possible YAML values ideally
as a regression test).

Ideally also this would be used by anaconda/bootc install to-disk
itself as it contains important information about the image
and ideally it would just need to encoded in a single place
in the bootc container.

---

osinfo: add support for /u/l/bootc-image-builder/disk.yaml

This commit implements reading of a custom per-container
partition table in /usr/lib/bootc-image-builder/disk.yaml.

The format is:

partition_table:
  type: "gpt"
  partitions:
    - &bios_boot_partition
      size: "1 MiB"
      bootable: true
      type: *bios_boot_partition_guid
      # "type: raw" is a planned feature, it does not exit yet
      payload:
        type: raw
        source: /usr/lib/firwmware/some-file.bin
    - &default_partition_table_part_efi
      size: "200 MiB"
      type: *efi_system_partition_guid
      payload_type: "filesystem"
      payload:
        type: vfat
...

i.e. the low-level disk package YAML.

---

distro: read partition table from bootc container

Read the partition table from the container via the osinfo
functionality. This will read the disk.yaml from the
previous commit and apply it if set as the default base
partition table.

[thozza]

Functionally, this looks good to me. I've added one question and some comments inline.

[thozza]

Thanks for the change. I agree that the code looks simpler now.

Unfortunately, I think you've missed adjusting the unit tests for it as well.

[thozza]

🚀

[alexlarsson]

This looks good to me