This change stores the digi pot indices in the scale 1-255
(previously 1 -128) so the stored values for calibration
are no longer valid after this change is uploaded to Arduino.
Please re-run the full calibration on your Afterburner!
This change handles EXTERNAL vs AR_EXTERNAL definitions of ADC constants
as defined on different Arduino platforms (Atmega vs Renesas).
Also an ADC gain error is compensated for Uno R4. The compensation
values are work-in-progress as I only have a single Uno R4, which
might no produce typical ADC values. That means Uno R4 needs more testing
reports while calibrating the VPP voltages.
New features:
* unified socket for 20 pin and 24 devices (including GAL20V8)
* variable VPP via digi pot
* VPP measurement via A0 ADC (used for VPP calibration)
