kernel: fix all nanokernel usage in comments

Also include kernel.h instead of nanokernel.h

Change-Id: I65dc5e31b5409b809397296817e2d5e7adf28892
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/include/nano_internal.h b/kernel/include/nano_internal.h
index 5396133..dfc4fbb 100644
--- a/kernel/include/nano_internal.h
+++ b/kernel/include/nano_internal.h
@@ -16,10 +16,9 @@
 
 /**
  * @file
- * @brief Architecture-independent private nanokernel APIs
+ * @brief Architecture-independent private kernel APIs
  *
- * This file contains private nanokernel APIs that are not
- * architecture-specific.
+ * This file contains private kernel APIs that are not architecture-specific.
  */
 
 #ifndef _NANO_INTERNAL__H_