[Toaster] Work on better artifact detection

[Smith, Elliot]

I have a version of the code for 8556 on this branch:

elliot/toaster/8556-image_fstypes

As it involves changes to toaster.bbclass, I would like to ask for feedback
on this branch as a whole, rather than submitting my patches to two
different mailing lists and having them potentially going out of sync.

To test, try building something with populate_sdk and/or populate_sdk_ext.

The trickiest part was figuring out how to get populate_sdk_ext artifacts,
as there's no direct link between the target you run and the files it
outputs. I fudged it by looking for files containing "-nativesdk-" in their
path: not ideal, but I was unable to figure out an alternative.

I also added a lot of code to clone artifacts from previous targets, if
those targets match the current target. For example, if you build
core-image-minimal, then build it again, the second build will clone the
artifacts from the first (as the second build won't create any image
files). (It's more complicated than that, so look at the code and comments
for details of the algorithm.)

This means that each target should always have artifacts associated with
it, even if it didn't produce them. Note that cloning of artifact records
between targets is only done within a project, and not done at all if a
target produced artifacts of its own.

As the code is fairly far-reaching, I would appreciate comments from as
many people as possible.

Thanks.
Elliot
-- 
Elliot Smith
Software Engineer
Intel Open Source Technology Centre
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