Copied from jcrocholl's contributions to the OpenBeamUSA Marlin fork

- Change some `previous_` time vars to `next_` so an add only happens
at intervals
- Fix `HEATER_0_USES_MAX6675` polling too frequently, or not at all

lcdDrawUpdate = 2 helped a lot!

- Prevent `Z_SAFE_HOMING` from homing Z twice in some cases
- Allow `G28` with XYZ values of 0 to explicitly set the position
- Don’t add `home_offset` when setting XYZ explicitly in `G28`
- Add `code_has_value` function to check for the presence of a numeric
value (could just test for space or nul to allow other types of values)

- MENU_ITEM causes lcd_quick_feedback to be called, so only those calls
to `lcd_goto_menu` outside of the `MENU_ITEM` macro need to set the
`feedback` flag when calling `lcd_goto_menu`.

- Replace “Mendel” with “RepRap”

- It’s inline here, but could be a macro and duplicated for
`planner.cpp`

Created a new board to make using RAMPS as a CNC machine controller
easier. It maps pin 8 to the spindle and pin 9 to the controller fan.

- Use `line_to_current` in place of `plan_buffer_line`

+ update in documentation

where the hardware is available.
Fix for #1873

The line const bool Z_PROBE_ENDSTOP_INVERTING = false; // set to true to
invert the logic of the endstop is in the main configuration.h but
wasn't present in the example configuration.h's. Causes a compiling
error with Z Probe enabled. Just added it to all the configs. :)

Thank you @Nprod!

Resolved some warnings in Marlin_main.cpp regarding compares of signed and unsigned integers by replacing 'int' or 'uint16_t' with 'uint8_t'.

to avoid errors in Marlin_main.cpp. #1860

In the include tree of Marlin_main.cpp the decision between the display types is not made.
To include the right LCD_STR_* ether 'dogm_lcd_implementation.h' or 'ultralcd_implementation_hitachi_HD44780.h' with all their code.
A 'dogm_lcd_implementation.h.h' would be a curiosity.
So i moved both of the definition blocks to conditionals.h

On the long term it could make sense to use the same numbering for the u8glib and the hitachi symbols.

and cleaned typo.

The other two devices we use in u8glib have more than only dummy code.
Checked this in the library code.

Squished #1849

- Set `de` to 0 when `position[E] = target[E]`
- Address regression mention in #1846