Release Marlin 2.0.8.1 release for Anycubic Mega Zero 1 and 2 · kad/Marlin

NOTES:

Please be careful with selecting right file to use:
- amz_ files are for Mega Zero version 1 (original Mega Zero, 12V power supply)
- amz2_ files are for Mega Zero V2.0 (24V power supply)
Boards:
- melzi -- original Anycubic board
- btt_mini -- BigTreeTech SKR mini MZ 1.0 or SKR mini E3 V2.0
- btt_e3turbo -- BigTreeTech SKR E3 Turbo

KAD Anycubic Mega Zero v1 and v2 unified configuration for upstream Marlin 2.0.8.1 release.

One code base with multiple PlatformIO build targets should be able to produce working firmware builds for Anycubic Mega Zero.

Build variants: see Wiki

Changes since v2.0.7.2-kad3:

Supported printers:
- Anycubic Mega Zero
- Anycubic Mega Zero 2.0
Supported boards:
- Anycubic Melzi
- BigTreeTech SKR mini MZ V1.0 / SKR mini E3 V2.0
- BigTreeTech SKR E3 Turbo
Leveling sensors:
- BLTouch
- BFPTouch
- PINDA
- Allenkey Probe
PSU control with BTT Relay 1.2 or MKS PWC
Fixed MAX Temp for bed (115) and hotend (245)
Defaults for bed PID calibration from MZ 2.0
Increase probing grid to 7x7
Probing margin 10mm
Enabled Subdivision of the grid by Catmull-Rom method (SKR boards)
PLA bed preheat at 60C
Use CR10_STOCKDISPLAY on all boards
Use smaller Marlin logo
Enabled M154 support
Board specific changes:
- Anycubic Melzi:
  - BLTouch/BFPTouch variants:
    - PIN27 + EXT-A2
    - PIN27 + Zmin
    - EXT-A2 + Zmin
  - Filament Sensors:
    - PIN27
    - EXT-A2
- BigTreeTech SKR (all):
  - Enable M73 support
  - Display remaining time
  - Power Loss function tailored for UPS modules
  - Binary file transfers
  - ARC support
  - Advanced Pause + filament load/unload
- BigTreeTech SKR E3 Turbo:
  - Dual extruder configs:
    - 2E: dual independant (Chimera)
    - 2to1: single nozzle, one filament at a time
    - 2mix: single mixing nozzle
  - UBL with BLTouch

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