sbuild schroots fail for older releases which don't support 'overlay' and only support 'overlayfs'

[Thomas Ward]

Environment: Trusty
Architecture: amd64
Kernel: lts-vivid
uname -a: Linux darksource 3.19.0-31-generic #36~14.04.1-Ubuntu SMP Thu Oct 8 10:21:08 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Previously, overlayfs was not available in schroot. It no longer errors on this.

However, according to Adam Conrad, in response to my asking what to do to prepare for my Trusty->Xenial update of my local environment with sbuild:
[2015-10-26 13:31:48] <infinity> teward: If you don't intend to downgrade from lts-v to 3.16 or 3.13 ever again, you can do that today.

Doing this makes everything fail for schroot under lts-vivid. Both mk-sbuild and sbuild itself error with "union-type: Unknown filesystem union type ‘overlay’" which suggests that it's not supported on Trusty. This may be a case of information being given to me incorrectly, but this is still a headache.

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Replication steps:

(1) Using any kernel that is older in Trusty, such as 3.13 or 3.16, have it configured for sbuild and schroots, thereby using 'overlayfs'.
(2) Use kernel lts-vivid then, so you have 3.19+
(3) Change the union-type to 'overlay' instead of 'overlayfs' for all schroots in /etc/schroot/chroot.d/*
(4) Attempt to use sbuild or mk-sbuild.
    (a) If you use sbuild to build a package, it will fail to mount saying something like Error 1 below, for any of your schroots. This will error for any and all schroots, regardless of the one in use or not (I was using a Xenial amd64 one, via `sbuild --arch=amd64 --dist=xenial ...` to build a source package).
    (b) If you use mk-sbuild you'll get a similar error at the end of the building of the schroot. The chroot is built fine, however the error still exists.

This is going to cause a headache in the future, but I guess I'm already starting to see some of these headaches.

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Error 1 (example):
E: /etc/schroot/chroot.d/sbuild-wheezy-i386: line 11 [wheezy-i386] union-type: Unknown filesystem union type ‘overlay’

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: sbuild 0.64.1-1ubuntu4
ProcVersionSignature: Ubuntu 3.19.0-31.36~14.04.1-generic 3.19.8-ckt7
Uname: Linux 3.19.0-31-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.16
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Oct 26 14:55:35 2015
InstallationDate: Installed on 2013-09-16 (770 days ago)
InstallationMedia: Ubuntu 12.04.3 LTS "Precise Pangolin" - Release amd64 (20130820.1)
PackageArchitecture: all
SourcePackage: sbuild
UpgradeStatus: Upgraded to trusty on 2014-10-22 (369 days ago)