We'd like layers to set this variable so that we know which layers are compatible
with which others, even if the branch is a generic un-updated "master" branch.
Start printing a warning to highlight this issue.
(Bitbake rev: cca81e33b58c390dcf5cc3a31555a43b79177166)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>