gateware-20230404

Hotfix for safe POT discharge

* Mapping of SID 2 to any or all of D420, D500, and DE00 (make SID2=1)
* Linting of the complete design with Verilator and Slang
* Configurable LFSR envelope counter logic
* Corrections to MOS8580 triangle waveform timing
* Safe POT discharge - thanks to @sorgelig for raising the issue