With version 1.5.3 you changed all keys from the old format `menu.cpu.<base-name>.<processor>.<other_keys>= <value>` to `<base-name>.menu.cpu.<processor>.<other_keys>= <value>` You missed to change line 102 in file hardware/arduino/avr/boards.txt it still reads: `menu.cpu.nano.atmega328.build.mcu=atmega328p`