meta-facebook: layer.conf: Add zeus compatibility

oe-core branched in preparation for the upcoming Yocto release.
We'll retain compatibility for warrior until we do our own branch
or stop testing warrior.

(From meta-facebook rev: fbbba83da1e5f2c2750fd5893aefc7a8439c72aa)

Change-Id: Ia459c224e0130aec85f27a5d8ff04bdae99a01e2
Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>