Add some configuration options to allow either the Analog reading of the FSR, or digital reading

Hi There

I have made a couple of changes.

1) Configuration.h has had a few defines added to allow either FSR-analog or FSR-digital.
2) qr_solve has had an include changed to allow compilation
3) Marlin_main has had the code cleaned up around reading the state of the bed touching in the auto-callibration.

Kind Regards

Tim Molteno