kernel: add closing comments to config endifs

Add a closing comment to the endif with the configuration
information to which the endif belongs too.
To make the code more clearer if the configs need adaptions.

Signed-off-by: Simon Hein <Shein@baumer.com>
diff --git a/kernel/mailbox.c b/kernel/mailbox.c
index 8e796a4..0d69076 100644
--- a/kernel/mailbox.c
+++ b/kernel/mailbox.c
@@ -22,7 +22,7 @@
 
 #ifdef CONFIG_OBJ_CORE_MAILBOX
 static struct k_obj_type  obj_type_mailbox;
-#endif
+#endif /* CONFIG_OBJ_CORE_MAILBOX */
 
 #if (CONFIG_NUM_MBOX_ASYNC_MSGS > 0)
 
@@ -97,7 +97,7 @@
 
 #ifdef CONFIG_OBJ_CORE_MAILBOX
 	k_obj_core_init_and_link(K_OBJ_CORE(mbox), &obj_type_mailbox);
-#endif
+#endif /* CONFIG_OBJ_CORE_MAILBOX */
 
 	SYS_PORT_TRACING_OBJ_INIT(k_mbox, mbox);
 }
@@ -189,7 +189,7 @@
 		}
 		return;
 	}
-#endif
+#endif /* CONFIG_NUM_MBOX_ASYNC_MSGS */
 
 	/* synchronous send: wake up sending thread */
 	arch_thread_return_value_set(sending_thread, 0);
@@ -256,7 +256,7 @@
 				z_reschedule(&mbox->lock, key);
 				return 0;
 			}
-#endif
+#endif /* CONFIG_NUM_MBOX_ASYNC_MSGS */
 			SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_mbox, message_put, mbox, timeout);
 
 			/*
@@ -286,7 +286,7 @@
 		k_spin_unlock(&mbox->lock, key);
 		return 0;
 	}
-#endif
+#endif /* CONFIG_NUM_MBOX_ASYNC_MSGS */
 	SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_mbox, message_put, mbox, timeout);
 
 	/* synchronous send: sender waits on tx queue for receiver or timeout */
@@ -335,7 +335,7 @@
 	(void)mbox_message_put(mbox, &async->tx_msg, K_FOREVER);
 	SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_mbox, async_put, mbox, sem);
 }
-#endif
+#endif /* CONFIG_NUM_MBOX_ASYNC_MSGS */
 
 void k_mbox_data_get(struct k_mbox_msg *rx_msg, void *buffer)
 {
@@ -463,4 +463,4 @@
 
 SYS_INIT(init_mailbox_obj_core_list, PRE_KERNEL_1,
 	 CONFIG_KERNEL_INIT_PRIORITY_OBJECTS);
-#endif
+#endif /* CONFIG_OBJ_CORE_MAILBOX */