I recently designed a board around the atmega2560 running at 3.3V and 8 MHz. I'd like to use the internal RC oscillator at 8 MHz. In the boards.txt file I added the following:
The only difference between this and the default mega2560 entry is that I've changed the build.f_cpu to 8 MHz and I've changed the low fuse to reflect the 8 MHz RC oscillator.
When I try to burn the bootloader I get the following error:
This is the error you usually see when you try to load a bootloader into a 2560 using a programmer like the USBTINY or ArduinoISP that doesn't really support more than 128k (64k words) of program space. Avrdude is smart enough to program the high memory anyway, but it does the verify wrong.
Your chip PROBABLY has an OK bootloader loaded at that point.
Note that unless you recompiled the bootloader, you haven't change the serial bitrate constants that it uses, and your upload will need to run at 57600bps instead of 115200bps. (you may find that the bootloader doesn't work at 115200bps even if you DO recompile...)