I have run out of flash memory and the bootloader must go in my Leonardo Ethernet board. I am at 106% of flash so that's ok, just burn it with a hardware ISP, right?
The problem is the Arduino IDE stops me because its too large despite trying to use a hardware ISP. I went looking for the boards.txt and there is a Leonardo in there but no Leonardo Ethernet.
Where is the Leonardo Ethernet definition where I can change it to the correct available memory?
You were just looking at the wrong version of boards.txt. It's installed to a different location when you update the Arduino AVR Boards package via Boards Manager. I suspected that was the issue and that's why I asked that question.
You will need to redo this change every time you update to a new version of the Arduino IDE or Arduino AVR Boards. You might consider creating a custom hardware package for your modified board so it will persist through updates. If that's something you want to do I can help.