classes: add workdir_save class

classes: add workdir_save class

If you are running your builds inside an environment where you don't
have access to the build tree (e.g. an autobuilder where you can only
download final artifacts such as images), then debugging build failures
can be difficult - you can't examine log files, the source tree or output
files. When enabled, this class triggers on task failure and saves a
tarball of the work directory for the task's recipe and puts it in a
configurable location, where it can be picked up by a separate process
and made available as a downloadable artifact.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.microsoft.com>