update Kconfig in raspi-64 bsp

 to fix RTT root path

bsp/raspberry-pi/raspi3-64/Kconfig

@@ -8,7 +8,7 @@ config BSP_DIR
 config RTT_DIR
     string
     option env="RTT_ROOT"
-    default "../.."
+    default "../../.."
 
 config PKGS_DIR
     string