kernel: Add k_mem_pool compatibility layer on top of k_heap

Add a shim layer implementing the legacy k_mem_pool APIs backed by a
k_heap instead of the original implementation.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/CMakeLists.txt

@@ -12,7 +12,6 @@ add_library(kernel
   mailbox.c
   mem_slab.c
   mempool.c
-  mempool_sys.c
   msg_q.c
   mutex.c
   pipes.c
@@ -42,6 +41,11 @@ target_sources_ifdef(CONFIG_SYS_CLOCK_EXISTS      kernel PRIVATE timeout.c timer
 target_sources_ifdef(CONFIG_ATOMIC_OPERATIONS_C   kernel PRIVATE atomic_c.c)
 target_sources_if_kconfig(                        kernel PRIVATE poll.c)
 
+if(${CONFIG_MEM_POOL_HEAP_BACKEND})
+else()
+  target_sources(kernel PRIVATE mempool_sys.c)
+endif()
+
 # The last 2 files inside the target_sources_ifdef should be
 # userspace_handler.c and userspace.c. If not the linker would complain.
 # This order has to be maintained. Any new file should be placed