If Marlin is blocking the serial input or command queue for any length
of time (for example more than 2 seconds), it needs to send a message
to serial out to inform the host that it is busy. Marlin should only
send these messages out when busy, and preferably not when trying to
print formatted output.

rebased
corrected spelling
changed to #elif for error

Still with MIN_Z_HEIGHT_FOR_HOMING but con be done with (folder wide) search-replace when we have a better name.

update old comments
update or delete deprecated definitions
complement forgotten updates
adjust spacing
some cosmetic changes

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

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.

Redo of #2939 rebased against RCBugFix

Renamed `WARN_REDUCED_ACCURACY` to `DISABLE_REDUCED_ACCURACY_WARNING`

Changed the condition for blinking from
```
#if ENABLED(WARN_REDUCED_ACCURACY)
```
to
```
#if DISABLED(DISABLE_REDUCED_ACCURACY_WARNING)
```

Replaced displaying "---" instead of the value of a coordinate when
unhomed or with reduced precision
with blinking the coordinate-prefix-character ('X','Y','Z').
For "unhomed" a '?' is shown every second second - until that axis is
homed. The value displayed is, as before the "---" where displayed, the
relative to the reset position coordinate value.
When the axis stepper was disabled, now we can display a hint on that,
by showing a blinking ' ' instead of the axis letter, when
WARN_REDUCED_ACCURACY is defined.

I suppose the code itself is here the better documentation.

A '+/-' character is in non of our charsets so i decided for a '?' for
now to reduce the work.
There is no additional space on the displays one could use to display
the information, so replacing something is the only option. As the axis
letters are totally redundant with their positions on the display they
contain the least information.
So my decision was to overwrite them.

Make stepper shutdown after inactivity dependent on a new set of
#defines.
DISABLE_INACTIV_X
DISABLE_INACTIV_Y
DISABLE_INACTIV_Z
DISABLE_INACTIV_E

And make exemplaric Configuration.
Names can be discussed.

This makes the disabling of the steppers independent from the DISABLE_?
settings witch shut down the steppers immediately.

This formalizes the dependency of MANUAL_BED_LEVELING on
MESH_BED_LEVELING

- Include aligned comment about new SDSUPPORT behavior

Add SDEXTRASLOW which runs the SD card at SPI_QUARTER_SPEED. Quarter speed is needed for RIGIDBOT_PANEL

With the introduction of the #if ENABLED(…)

reads better than

* Updates the auto bed leveling comments ONLY.
* Renames "Z probe" word usage in //comments and #errors properly.

Currently we use the probe exclusively as a device to find the build platform(bed).
For the currently supported setups this means, we use it as a additional min-endstop.
A triggered when not deployed probe disturbs the homing process for max-endstops.

Rename ENDSTOPPULLUP_ZPROBE to ENDSTOPPULLUP_ZMIN_PROBE
Rename Z_PROBE_ENDSTOP_INVERTING to Z_MIN_PROBE_ENDSTOP_INVERTING
Rename Z_PROBE_ENDSTOP to Z_MIN_PROBE_ENDSTOP
Rename DISABLE_Z_PROBE_ENDSTOP to DISABLE_Z_MIN_PROBE_ENDSTOP
Rename Z_PROBE_REPEATABILITY_TEST to Z_MIN_PROBE_REPEATABILITY_TEST
Rename Z_PROBE_ENDSTOP to Z_MIN_PROBE_ENDSTOP
Adjust comments accordingly
Remove Z_MAX  check for the probe in update_endstops().

Using an delta related idea of @clefranc from #61,
extended to the general change for all setups.

Tested with Prusa i3, max-z-endstop and permanently triggered z-probe.
Worked for @clefranc's delta.

The long descriptor will still be available in M115

Uses about 488 byte of progmem and 32 byte of RAM.
For that configurable.

- For some reason it’s easy to forget what BT stands for, but it
doesn’t need an abbreviation.

- `SD_DETECT_PIN` replaces `SDCARDDETECT`
- `SD_DETECT_INVERTED` replaces `SDCARDDETECTINVERTED`
- Revise the description of `SD_DETECT_INVERTED`
- Add a note about the override of `SD_DETECT_INVERTED` in
`Conditionals.h`

A speaker needs a AC or a pulsed DC to make a sound, a buzzer only needs a DC.
A buzzer has it's own resonator. It works in most cases to feed the buzzer with a pulsed DC, but the sound will not be as loud as with pure DC.

There seem to be boards where the BEEPER-pin is not able to handle a PWM. Obviously intended for a buzzer.
To make these board able to handle a speaker

* replace the PWM based tone()-function again with a on-delay-off-delay loop.

Hopefully the last time I touch the beeper code.

Just set up the pin. Don't move to a random position.

Simplify servo::move()
* servo::move() does not need the pin parameter - The pin is set during servo_init() with attach().
* servo::move() does not need a return value.

SERVO_LEVELING is the wrong condition to deactivate the servos.

Remove some temporary (Servo *) variables.
SanityCheck for the servo indexes.