kernel: Refactor k_mem_pool APIs into a base and derived level

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>

kernel/CMakeLists.txt

@@ -11,6 +11,7 @@ add_library(kernel
   mailbox.c
   mem_slab.c
   mempool.c
+  mempool_sys.c
   msg_q.c
   mutex.c
   pipes.c