Allow custom root labels

Makes the localboot element aware of the custom root fs labels
added in I596104d1a63b5dc6549e8460a1ae3da00165ef04 for
diskimage-builder.  That change must merge before this one.

Change-Id: Ic89379eca8699b27a4b89de9da3fc4f56011963e
Merge "Allow custom root labels"