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