Fix incorrect Yun extended fuse

Author: facchinm

hardware/arduino/avr/boards.txt

@@ -28,7 +28,7 @@ yun.upload.wait_for_upload_port=true
 yun.bootloader.tool=avrdude
 yun.bootloader.low_fuses=0xff
 yun.bootloader.high_fuses=0xd8
-yun.bootloader.extended_fuses=0xfb
+yun.bootloader.extended_fuses=0xcb
 yun.bootloader.file=caterina/Caterina-Yun.hex
 yun.bootloader.noblink=caterina/Caterina-Yun-noblink.hex
 yun.bootloader.unlock_bits=0xFF