Marlin 2.0.9 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
  • btt_e3turbo -- BigTreeTech SKR E3 Turbo

KAD Anycubic Mega Zero v1 and v2 unified configuration for upstream Marlin 2.0.9 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

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

Changes since v2.0.8.2-kad1:

• Enable HOST_ACTION_COMMANDS, issue #19
• BTT SKR mini boards are using older and stable Maple based STM32F1 HAL.
  Few STM32 HAL based targets added for testing, but not yet fully tested and not provided as binaries.