Implemented and tested for both platforms. This is quite a big
commit. Unlike with the previous changes to now choosable PWM
frequencies, all board configuration files and Configtool had
to be changed immediately to deal with the additional parameter
in DEFINE_HEATER() and keep AVR builds working (and regression
tests passing).
These were dropped accidentally, I assume, during the Configtool
template conversion. Without them the INTERCOM feature does not
compile and so board.gen3.h is broken.
Add the gen3 config to testcases and regressiontests to ensure
this does not drop again.