code changes to implement G38 commands

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

Marlin/Marlin.h

@@ -209,6 +209,12 @@ void manage_inactivity(bool ignore_stepper_queue = false);
 
 #endif // !MIXING_EXTRUDER
 
+#define G38_2_3
+#ifdef G38_2_3
+extern bool G38_flag ;   //flag to tell the interrupt handler that a G38 command is being run
+extern bool G38_flag_pass ;   //flag from the interrupt handler to indicate if the endstop went active
+#endif
+
 /**
  * The axis order in all axis related arrays is X, Y, Z, E
  */