[Merge] ~toabctl/livecd-rootfs:boot-partition into livecd-rootfs:ubuntu/master

[Philip Roche]

Review: Approve

I am +1 given that we have noted many of the test results - however we are having issues testing s390x ( https://rt.admin.canonical.com/Ticket/Display.html?id=159646 - link internal to Canonical only) which would hinder verifying this change. 
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https://code.launchpad.net/~toabctl/livecd-rootfs/+git/livecd-rootfs-1/+merge/451330
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