And reduce the function size to simplify

.. and give the hint to try lower values if possible.

Simply apply enabled settings instead of requiring users to edit
stepper_indirection.cpp

lcd_update can take so much time that the block buffer gets drained if
there are only short segments. This leads to jerky printer movements for
example in circles and a bad print quality.

This change implements a simple check: Only if the block currently
executed is long enough, run lcd_update.
This also means the printer will not show actual values on the LCD nor
will it respond to buttons pressed. A option that keeps the menu
accessible is also available.
Aditionaly, slow down if a block would be so fast that adding a new
block to the buffer would take more time. In this case, the buffer would
drain until it's empty in worst case.

・Update forgotten K8400\Dual-head\Configuration.h
・Adjust spacing

implemented G-Code M355 into Marlin

With this formula, a start value for K for further fine-calibration can be calculated.

1) modified 3 code files
Marlin.h
Marlin_main.cpp
endstops.cpp

2) modified config files so I could test on my machine

Testing was done on an AzteegX3pro based machine.

The probe was hooked to the Z_MIN endstop.

My controller doesn't have a dedicated Z_PROBE input so I couldn't test
that functionality.

Verified that a large file (without any G38 commands) executed the same
before and after the changes.

Verified that the head moves as expected when G38.2 and G38.3 commands
are issued. Single & multiple axis moves were tested along with + and -
directions.

Code was added to the main ISR. In normal operation only one extra IF
statement is evaluated. I didn't notice any performance degradation
because of the added code.

The G38 commands are expected to be issued manually by the operator
during machine setup. The G38 commands wait until the machine is idle
before proceeding. That way the other commands are minimally impacted
by the extra ISR overhead when a G38 command is in the queue.

The G38 commands are very similar to the G28 commands except 1) only the
Z_PROBE is used and movement can be in the + or - direction.

See issue 4677 for a discussion on adding G38 commands to Marlin.
Feature request: add ability to use G38.2 command (CNC)
MarlinFirmware/Marlin#4677

About Configuration.h:
・Fix the PR #4899 (ABL: Enable by type. Bilinear for all.)
Remove Duplicated contents

・Fix the PR #4305 (Custom boot screen feature improvement)
Revert from "during boot" to "during bootup" in all the example
Configuration.h

・Fix the PR #4207 (Clean up, simplify and generalize the Allen-key-probe
code.)
Resolve and relocate the duplicated definitions in Z_PROBE_ALLEN_KEY
section

・Follow-up the PR #4805 (Additional documentation of Configuration.h)
Add forgotten changes to all the example Configuration.h

Adjust spacing

About Configuration_adv.h:
Add missing description of SLOWDOWN for DELTA

Adjust spacing

Follow-up to #4666

Rather than include twice, which is tricky but may confuse

Similar to the current Arduino HardwareSerial
but with max. 256 byte buffer-size.

Deactivated by default.

The boards with AT90USB processor (USBCON) already use a TX-buffer.

Add an emergency-command parser to MarlinSerial's RX interrupt.

The parser tries to find and execute M108,M112,M410 before the commands disappear in the RX-buffer.

To avoid false positives for M117, comments and commands followed by filenames (M23, M28, M30, M32, M33) are filtered.

This enables Marlin to receive and react on the Emergency command at all times - regardless of whether the buffers are full or not. It remains to convince hosts to send the commands. To inform the hosts about the new feature a new entry in the M115-report was made. "`EMERGENCY_CODES:M112,M108,M410;`".

The parser is fast. It only ever needs two switch decisions and one assignment of the new state for every character.

One problem remains. If the host has sent an incomplete line before sending an emergency command the emergency command could be omitted when the parser is in `state_IGNORE`.
In that case the host should send "\ncommand\n"

Also introduces M108 to break the waiting for the heaters in M109, M190 and M303.

Rename `cancel_heatup` to `wait_for_heatup` to better see the purpose.

Includes:
  *firmware version / branch / date.
  *extruder count
  *board information (name, serial details, power supply type)
  *thermistors (names, min/max temperatures)
  *printer statistics (PRINTCOUNTER details)

Thanks to @thinkyhead for contributions.