Marlin 2.0.8.2 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 2.0.8.1 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

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

Changes since v2.0.8.1-kad1:

• In MZ1 + Heated bed configuration, minimal and maximum X/Y positions are adjusted on assumptions of 220x220 Ultrabase bed upgrade.
• On SKR boards, new default baud rate is 250000. If you're using BTT TFT, you might need to update connection speed in TFT settings.