paging: Rename existing CONFIG_PAGING to CONFIG_LEGACY_PAGING

Current `CONFIG_PAGING` refers to an experimental implementation
to enable embedded MCUs with some limited RAM space to execute
large programs from some non-random access media.

On-demand paging should be implemented for the kernel mode with
address environment implementation enabled.

include/nuttx/page.h

@@ -33,7 +33,7 @@
 #  include <nuttx/sched.h>
 #endif
 
-#ifdef CONFIG_PAGING
+#ifdef CONFIG_LEGACY_PAGING
 
 /****************************************************************************
  * Pre-processor Definitions
@@ -455,5 +455,5 @@ int up_fillpage(FAR struct tcb_s *tcb, FAR void *vpage,
 #endif
 
 #endif /* __ASSEMBLY__ */
-#endif /* CONFIG_PAGING */
+#endif /* CONFIG_LEGACY_PAGING */
 #endif /* __INCLUDE_NUTTX_PAGE_H */

include/nuttx/sched.h

@@ -226,7 +226,7 @@ enum tstate_e
   TSTATE_WAIT_MQNOTEMPTY,     /* BLOCKED      - Waiting for a MQ to become not empty. */
   TSTATE_WAIT_MQNOTFULL,      /* BLOCKED      - Waiting for a MQ to become not full. */
 #endif
-#ifdef CONFIG_PAGING
+#ifdef CONFIG_LEGACY_PAGING
   TSTATE_WAIT_PAGEFILL,       /* BLOCKED      - Waiting for page fill */
 #endif
 #ifdef CONFIG_SIG_SIGSTOP_ACTION