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).
Also move #defines from heater.c to heater.h
This operation became bigger than expected, because PID handling
hat to be separated from heater handling. Code and strategy wasn't
changed, but some chunks of code had to be moved.
Markus Hitter