[2.0.x] Z_PROBE_LOW_POINT to prevent probe/bed damage

See https://github.com/MarlinFirmware/Marlin/issues/10317#issuecomment-379100702

As suggested by @GMagician

Responding to https://github.com/MarlinFirmware/Marlin/pull/8833#issuecomment-379426803

Based on #10343 by @indazoo

[2.0.x] 6th-order jerk-controlled motion planning in real-time

Enable 6th-order jerk-controlled motion planning in real-time.
Only for 32bit MCUs. (AVR simply does not have enough processing power for this!)

[2.0.x] Monitor chamber temperature, with auto fan control.

This is an initial cut for feedback, updated for 2.0.x.

Chamber temperature is currently reported along with hot end and bed
temperatures to serial. The format is just like that used for hot end
and bed temperatures, but using 'C' prefix. As there is no heater,
target is always 0. Is this appropriate, is there a better way to report
chamber temperatures?

Chamber temperatures are not reported on the LCD in any way.

When auto chamber fan is enabled, it currently just uses the same
temperature threshold as the other auto controlled fans.

As the chamber temperature is not connected to any heater, it doesn't
undergo mintemp/maxtemp monitoring. This would need to change in the
future if chamber heating became a feature.

Based on #10313 by @rmoravcik

[2.0.x] Fix TMC2130 DIAG1 active high, Einsy Retro

Fix #10330

Updated to #10296

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