Almost all of the k_mem_pool API is implemented in terms of three
lower level primitives: K_MEM_POOL_DEFINE(), k_mem_pool_alloc() and
k_mem_pool_free_id(). These are themselves implemented on top of the
lower level sys_mem_pool abstraction.
Make this layering explicit by splitting the low level out into its
own files: mempool_sys.c/h.
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
