boards.txt: fixed wrong extended fuses when using external programmer with a yun

Author: Federico Fissore

avr/boards.txt

@@ -23,7 +23,7 @@ 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=0xcb
+yun.bootloader.extended_fuses=0xfb
 yun.bootloader.file=caterina/Caterina-Yun.hex
 yun.bootloader.unlock_bits=0x3F
 yun.bootloader.lock_bits=0x2F