I duplicated the entry for ATMEGA328P, then replaced the relevant lines that you posted in avrdude.conf. Then, in boards.txt, I changed the following line:
atmega328.build.mcu=atmega328p
to
atmega328.build.mcu=m328au
and now when I try to upload, I get an error saying that the stk500 programmer is not responding.
I'm curious, where do you get the .build.mcu identifier from? At first I thought it was the "id" field of the chip you want from avrdude.conf, but when I search that file for atmega328p (the original identifier), it does not match any 'id' field, only a 'desc' field.