uboot-envtools: add Arduino Yun support

Signed-off-by: Camille Bilodeau <[email protected]>

package/boot/uboot-envtools/files/ar71xx

@@ -19,6 +19,7 @@ a60|\
 alfa-ap120c|\
 all0258n|\
 ap90q|\
+arduino-yun|\
 cap324|\
 cap4200ag|\
 carambola2|\