[Openembedded-architecture] RFC: RM_OLD_IMAGE as default behaviour

[Joshua Lock]

tl;dr: RM_OLD_IMAGE behaviour is effectively enabled by default in
master/morty — any objections?

Around a month ago a change landed in OE-Core to put image deployment
under sstate control[1]. 

This had a side-effect of removing old images from DEPLOY_DIR_IMAGE
when a new image is built[2], effectively making RM_OLD_IMAGE[3] the
default behaviour and making that variable redundant.

Several of us run with RM_OLD_IMAGE and didn't notice the change in
behaviour, others noticed and assumed it was an intended side-effect of
the change. 

This is arguably a significant behaviour change so I wanted to ensure
it was more visible before we tag a release with this behaviour.

Do folks have strong opinions on whether we should revert to the
previous behaviour of not removing images (also sdk's and toolchains)
when deploying a new version, or whether we should keep the new
behaviour, document it and remove the RM_OLD_IMAGE variable? Or perhaps
something else entirely?

Thanks,

Joshua

1. http://git.openembedded.org/openembedded-core/commit/?id=d54339d4b1a
7e884de636f6325ca60409ebd95ff
2. https://bugzilla.yoctoproject.org/show_bug.cgi?id=10419
3. http://www.yoctoproject.org/docs/2.1/ref-manual/ref-manual.html#var-
RM_OLD_IMAGE