Marlin 2.0.9.1 release for Anycubic Mega Zero 1 and 2

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, new STM32 HAL.
  • btt_mini_maple -- BigTreeTech SKR mini MZ 1.0 or SKR mini E3 V2.0, builds are using older (pre-2.0.9) STM32F1 HAL.
  • btt_e3turbo -- BigTreeTech SKR E3 Turbo

KAD Anycubic Mega Zero v1 and v2 unified configuration for upstream Marlin 2.0.9.1 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

For usage of different leveling sensors, please read Leveling Sensors wiki page

Flashing: please read Wiki to find right instructions for your environment

If you want to use VSCode to rebuild from source, refer to wiki page to see screenshot on how to select your desired build target.

Changes since v2.0.9-kad1:

• BTT SKR mini boards are now using new upstream default settings with STM32 HAL
• If you want to use BTT SKR mini boards with older and stable Maple based STM32F1 HAL, use btt_mini_maple tagged builds.
• NO_VOLUMETRICS on BTT SKR mini builds to save space due to new STM32 HAL.