http://reprap.org/wiki/EinsyRetro

- Added sanity check to inform users to set the endstop to inverting
  when using SENSORLESS_HOMING

From #10311 by @anton-piliugin

[2.0.x] Continuous Purge

Followup to #10257

* Fixed Makefile for Marlin 2.0 (#10255)

- Makefile now supports the new Marlin 2.0 directory hierarchy.
- RELOC_WORKAROUND is now automatically enabled based on avr-gcc version.

* Makefile support for U8glib and TMC2130Stepper

- Updated paths for oliver's U8glib 1.19.1
- Added option for teemuatlut's TMC2130Stepper 2.2.1

Fixes #10278

[2.0.x] Fix Ender 4 compilation, add more AVRs to Makefile

- Assign 243 (like EFB) as Ender-4/CR-8 board ID
- Add to `Makefile`
- Add to `pins.h`

Addressing #10200

Fix #10256

Do the Z lift normally before setting Z back to its prior value. But do the Z lower using spoofing. This should produce proper symmetrical movement.

This ensures that steppers that share an enable pin will only be disabled for a miniscule time.

See https://github.com/MarlinFirmware/Marlin/issues/10246#issuecomment-377146936

Pending a more complete solution to tool-change Z deviation.

Followup to #10250

[2.0.x] Improvements to general and delta homing