meta-yocto: Rename to meta-poky to better match its purpose

"poky" is the reference distribution for the Yocto Project. This renames
the layer within the meta-yocto repository to meta-poky, better matching
what that layer contains.

A layer.conf file is left behind as this is the only way which allows
existing builds to migrate safely to the new name. It will be removed
at some future point.

This change requires the corresponding OE-Core change to handle the
migration and the changes to the infrastructure to support this.

(From meta-yocto rev: d0c88df2e14672fca4ebbde93c5efbcd0e4fa9b6)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>