Okay, find your boards.txt file.
You will see that every section has a line like this:
atmega168.build.mcu=atmega168
You will not find one that end with =atmega328
That name is what ties in with a similar section in avrdude.conf to tell the programmer what it is dealing with.
So maybe you add this to your boards.txt file:
##############################################################
uno_noP.name=Arduino Uno for Atmega328
uno_noP.upload.protocol=arduino
uno_noP.upload.maximum_size=32256
uno_noP.upload.speed=115200
uno_noP.bootloader.low_fuses=0xff
uno_noP.bootloader.high_fuses=0xde
uno_noP.bootloader.extended_fuses=0x05
uno_noP.bootloader.path=optiboot
uno_noP.bootloader.file=optiboot_atmega328.hex
uno_noP.bootloader.unlock_bits=0x3F
uno_noP.bootloader.lock_bits=0x0F
uno_noP.build.mcu=atmega328
uno_noP.build.f_cpu=16000000L
uno_noP.build.core=arduino
uno_noP.build.variant=standard
avrdude.conf has a 328P section, but not 328 - so you need to copy/paste that section & make a 328 section so boards.txt can pull in the needed info.
Here's the top of it and what you need to change, the rest of the section gets copied as-is.
#------------------------------------------------------------
# ATmega328P <<< remove the P
#------------------------------------------------------------
part
id = "m328p"; // <<< remove the P
desc = "ATMEGA328P"; // <<< remove the P
has_debugwire = yes;
flash_instr = 0xB6, 0x01, 0x11;
eeprom_instr = 0xBD, 0xF2, 0xBD, 0xE1, 0xBB, 0xCF, 0xB4, 0x00,
0xBE, 0x01, 0xB6, 0x01, 0xBC, 0x00, 0xBB, 0xBF,
0x99, 0xF9, 0xBB, 0xAF;
stk500_devcode = 0x86;
# avr910_devcode = 0x;
signature = 0x1e 0x95 0x0F; // <<< change these to the '328 bytes - maybe just 0F to 14??