Just a quick hint, didn't see this anywhere in the documentation or in the forums, so hope this helps.
Reference (20MHz core):
When you have a piece of custom hardware in your <sketch folder>/hardware/<board type>/boards.txt
, sometimes you just want to use an existing Arduino core. Documentation hints about the <board name>.build.core=<board type>:<core folder>
format, but you also need to do the same thing for <board name>.build.variant=<board type>:<variant folder>
to allow it to find the correct pins_arduino.h file.