nds32: delete unused local variable

Fixes the build-time warning:
	board.c: In function 'board_init_r':
	board.c:304: warning: unused variable 's'

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/nds32/lib/board.c

@@ -301,7 +301,6 @@ void board_init_f(ulong bootflag)
  */
 void board_init_r(gd_t *id, ulong dest_addr)
 {
-	char *s;
 	bd_t *bd;
 	ulong malloc_start;