ubuntu-image 0.12

[Barry Warsaw]

ubuntu-image 0.12 has been released to the edge channel snap store and zesty,
with SRUs to yakkety and xenial pending.  This fixes several reported problems
and adds some new features.  In particular:

LP: #1645750 gives you a new structure key `type: bare` which let's you define
a structure that isn't wrapped in a disk partition.

LP: #1642999 improves on the structure sort ordering.  Now structures which
are wrapped in partitions get their partition numbers from the order in which
they appear in the gadget.yaml.

LP: #1617421 provides more human readable error output, with all the gory
details hidden behind --debug.

LP: #1638660 deprecates `type: mbr` structure keys.  Use the new `role` key
instead or use `type: bare` if you've been using `type: mbr` to get around
partition wrapping.  At some point `type: mbr` will go away.

Please let us know how it goes for you so we can promote edge to beta (our
"released" channel since ubuntu-image runs in devmode).

Cheers,
-Barry