sfdisk is a bit crusty - it doesn't understand gpt partition tables very well,
for example. By switching to parted, we can handle gpt issues (which may be
useful in the future, and is definitely useful for other boards), and we no
longer have to hardcode that 4M alignment workaround. Parted will tell us
the free space at the end of the disk.
Because we're already using partprobe, there's no additional dependencies
needed.
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