sgdisk and sfdisk blockers

[Barry Warsaw]

We use sgdisk and sfdisk to manipulate partitions, but these tools have several known limitations.

* We can't set offset=0 for a non mbr-type partition.
* We can't name change partition names for mbr-schema partitions
* We can't create partitions with alignments other than 1MiB (see LP: #1621151)

These are now fairly significant blockers for moving forward with ubuntu-image.

[Barry Warsaw (barry)]

I'm unmilestoning this issue from 0.12 since we won't fix/eliminate sgdisk/sfdisk for this release.

[Barry Warsaw (barry)]

Actually, based on further discussion, I'm closing this issue. Item 1 is nonsensical and really was a problem related to the abuse of the special type=mbr, which we've solved with role=bare. Item 2 was an implementation issue caused by unconditionally trying to use the volume key name in the gadget.yaml as the partition name, regardless of the schema. Now what we do there is basically just ignore the `change-name` option when trying to partition an mbr schema disk. It's ugly but workable.
We'll just deal with item 3 with its own existing LP: #1621151