meta-zephyr-core/bsp: Restructure into sublayers

This commit restructures meta-zephyr into meta-zephyr-core and
meta-zephyr-bsp. It moves machine definitions into meta-zephyr-bsp
in preparation for adding the autogenerated zephyr machines.

Signed-off-by: Eilís Ní Fhlannagáin <elizabeth.flanagan@huawei.com>
Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>