Ah ah non me n'ero accorto 
In pratica nel file boards.txt, nella sezione dedicata all'arduino ethernet, ho modificato in questo modo
ethernet.name=Arduino Ethernet (with USB2Serial module)
#ethernet.upload.protocol=stk500
ethernet.upload.protocol=arduino
ethernet.upload.maximum_size=32256
ethernet.upload.speed=115200
ethernet.bootloader.low_fuses=0xff
ethernet.bootloader.high_fuses=0xde
ethernet.bootloader.extended_fuses=0x05
ethernet.bootloader.path=optiboot
ethernet.bootloader.file=optiboot_atmega328.hex
ethernet.bootloader.unlock_bits=0x3F
ethernet.bootloader.lock_bits=0x0F
ethernet.build.mcu=atmega328p
ethernet.build.f_cpu=16000000L
ethernet.build.core=arduino
ethernet.build.variant=standard
in pratica al posto di:
ethernet.upload.protocol=stk500
ho messo:
ethernet.upload.protocol=arduino