classes: add new retain class for retaining build results

classes: add new retain class for retaining build results

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, by default this class will retain the work
directory for any recipe that has a task failure in the form of a
tarball, and can also be configured to save other directories on failure
or always.

It puts these tarballs in a configurable location (${TMPDIR}/retained by
default), where they can be picked up by a separate process and made
available as downloadable artifacts.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>