EEPROM function and small Changes

[midopple]

• Changed homing function to not conflict with min_software_endstops/max_software_endstops
• Changed check in arc_func
• Corrected distance calculation.
• MAX Feed Rate for Z-Axis reduced to 2 mm/s some Printers had problems with 4 mm/s
  • the microcontroller can store settings in the EEPROM
  • M500 - stores paramters in EEPROM
  • M501 - reads parameters from EEPROM
  • M502 - reverts to the default "factory settings".
  • M503 - Print settings