- Add a new parameter `imageType` that can specify either "efi" or
"legacy" (the default which should see no change in behaviour by
this patch).
- EFI images get a GPT partition table (instead of msdos) with a
mandatory ESP partition (so we add an assert that `partitioned`
is true).
- Use the partx tool from util-linux to determine exact start + size
of the root partition. This is required because GPT stores a secondary
partition table at the end of the disk, so we can't just have
mkfs.ext4 create the filesystem until the end of the disk.
- (Unrelated to any EFI changes) Since we're depending on the
`-E offset=X` option to mkfs which is only supported by e2fsprogs,
disallow any attempts of creating partitioned disk images where
the root filesystem is not ext4.
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