Add runtime axis and homing direction control

[ellensp]

Description

This PR adds two optional runtime tuning features:

• RUNTIME_AXIS_DIRECTION: Change motor direction at runtime with G-code (M670) instead of rebuilding firmware.
• RUNTIME_HOMING_DIRECTION: Change homing direction at runtime with G-code (M671), including matching endstop behavior when both min/max pins exist.

Both settings are saved to EEPROM, so changes persist after restart. A reset (M502) restores defaults from configuration.

Requirements

• Requires enabling RUNTIME_AXIS_DIRECTION and/or RUNTIME_HOMING_DIRECTION in Configuration_adv.h.
• Runtime endstop pin swapping applies only on boards that define both MIN and MAX pins for an axis.

Benefits

• Makes machine setup and troubleshooting much faster.
• Reduces compile/flash cycles when reversing motors or testing homing direction.
• Keeps behavior flexible while still preserving a safe default from config.

Configurations

• Tested with mega2560 build.
• Suggested validation:
  • Use M670 to flip an axis direction and confirm movement changes.
  • Use M671 to switch homing direction and confirm homing/endstop behavior.
  • Save/reload with M500 / M501, then verify reset behavior with M502.

Related Issues

Feature request: runtime control of axis direction and homing direction without reflashing.

[ellensp]

This has not been tested on real hardware at all yet. Just wanted to make it public for constructive feedback.

The G-codes were picked at random. I have no problems if they get changed

[Nuck-TH]

I'd suggest to use RRF gcodes, so incompatibility doesn't grow.

[ellensp]

M569 https://reprap.org/wiki/G-code#M569:_Stepper_driver_control is quite different and extensive...
I cant find a homing dirction gcode

[Nuck-TH]

https://reprap.org/wiki/G-code#M574:_Set_endstop_configuration