Related to MarlinFirmware/MarlinDev#270

See: https://github.com/Ultimaker/Marlin/pull/7

As suggested by @max-pau in #3073 - #3074

files.

This changes have being discussed at MarlinFirmware/Marlin#2849.

Disallowed the Z_PROBE_OFFSET_RANGE_MAX to be greater than zero.

 - Remove the forced activation of SDSUPPORT, I guess people may use the ZUM
3D board and not the LCD module.
 - Fixed the tab issue within boards.h file.

Added support for BQ ZUM Mega 3D board and stock Hephestos 2 thermistor.

- Defining `SOURCE_CODE_URL` in `Default_Version.h` overrides it
elsewhere. So don’t define it there.
- Also fix up `language.h` so no `#undef` will be needed.

Addressing issue #2767

Apply `constrain`, `NOMORE`, `NOLESS` and `CRITICAL_SECTION` macros
wherever possible.

Fix layout descr. in configuration_store.cpp

Enhanced #3010 - formal Mesh/Manual dependency

Rc bug fix - Fix for issue #2931

Add CORE support to st_get_position_mm (and elsewhere)

Fix Printrboard with LCD (redo)

Interrupt safe serial --- Guard against non-atomic variable changes from interrupt routines

Based on #2860 - rebased for RCBugFix

Pre home display - Stepper release

Apply bed leveling matrix when switching extruders

Various minor spelling corrections

Redo of #2939 rebased against RCBugFix

Correct raise_z_for_servo for Z offsets below the nozzle

Fix status LEDs update when reinitializing the display

Add watchdog reset in the kill loop to simplify recovering.

Remove unused ALARM_PIN

Fix disable_all_heaters() for MAX6675

Remove unused ALARM_PIN from all the Configuration.h's

Enable USE_WATCHDOG by default

Code cleanup in gcode_T