Building projects on the command line using the Bare-Arduino-Project template I'm trying to program an Adafruit Trinket using avr-gcc et. al. with the Makefile from the Bare-Arduino-Project on macOS Catalina.

I can build projects for standard boards like the Uno, but not for the Trinket. Everything works as expected using the "official" Arduino IDE (1.18.10). In the Bare-Arduino-Project Makefile I'm specifying the Trinket as:

BOARD_TAG         = trinket5
BOARD_SUB         = attiny85


I get an error from avr-gcc that mmcu is not defined. I assume this is because the command line tools aren't finding the Adafruit equivalent of boards.txt. When I look in the Arduino IDE package I don't find the Adafruit boards in boards.txt or anything that looks like the definitions for the Adafruit boards (despite having added them in the IDE).

Any ideas about how to set up support for the Trinket (or other third party boards) when using command line tools?
You  probably look at the wrong boards.txt:

# Trinket 5V Configuration
trinket5.name=Adafruit Trinket (ATtiny85 @ 16MHz)


this can be found in the boards.txt in the main directory

C:\Users\YOUR_USER_NAME\AppData\Local\Arduino15\packages\arduino\hardware


in the sub directory 

\adafruit\avr 

in the same main directory should  also be a folder named 1.8.2 (for ArduinioIDE 1.8.12) for the core files

if added via board-manager using this line in preferences/additional bord manager urls

https://adafruit.github.io/arduino-board-index/package_adafruit_index.json


If you did a manual installation check with this guide for pit traps.Details on using trinket with the avrdude command line can be found here