Restore simulation build target

This code was accidentally removed long ago in a botched merge.  This
patch recovers it and makes it build again.  I've done minimal testing
and some necessary cleanup.  It compiles and runs, but it probably still
has a few dust bunnies here and there.
Add M203 heater PID monitoring

RepetierHost has a nice graph to assist in PID tuning.  It is enabled
with M203.
Loose ends; probably unneeded