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

Don't check Z_MAX on raise when the probe pwns the pin

Fix up ABL - Add PROBE_Y_FIRST option. Arduino 1.6.8 required.

K8400 has 1xXY an 2xZ endstop plugs

Rigidboard differs a little in configuration from the PrinterBoardRevF we modeled the code after.

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

- Extrapolate properly for even numbered grid points
- Extrapolate using average, not median
- Improve bilinear grid report output
- Add debug output for bilinear extrapolation
- Add option to extrapolate from edge, not center

ABL: Enable by type. Bilinear for all.

Define status LED pins without condition

Include requirements in "Implemented Codes"

Fix HEATERS_PARALLEL