Introduce axis_homed

Introduce additional variable axis_homed to replace axix_known_position
when the coordinate display should indicate the axis is not homed.
This is to distinguish between "not homed" and "inexact position possible
because stepper was disabled".

# Conflicts:
#	Marlin/ultralcd_implementation_hitachi_HD44780.h
solved

Marlin/Marlin.h

 extern float min_pos[3]; // axis[n].min_pos
 extern float max_pos[3]; // axis[n].max_pos
 extern bool axis_known_position[3]; // axis[n].is_known
+extern bool axis_homed[3]; // axis[n].is_homed
 
 #if ENABLED(DELTA)
   extern float delta[3];

Marlin/Marlin_main.cpp

 float current_position[NUM_AXIS] = { 0.0 };
 static float destination[NUM_AXIS] = { 0.0 };
 bool axis_known_position[3] = { false };
+bool axis_homed[3] = { false };
 
 static long gcode_N, gcode_LastN, Stopped_gcode_LastN = 0;
 
     feedrate = 0.0;
     endstops_hit_on_purpose(); // clear endstop hit flags
     axis_known_position[axis] = true;
+    axis_homed[axis] = true;
 
     #if ENABLED(Z_PROBE_SLED)
       // bring Z probe back

Marlin/dogm_lcd_implementation.h

   u8g.drawPixel(8, XYZ_BASELINE - 5);
   u8g.drawPixel(8, XYZ_BASELINE - 3);
   u8g.setPrintPos(10, XYZ_BASELINE);
-  if (axis_known_position[X_AXIS] || (blink & 1))
+  if (axis_homed[X_AXIS] || (blink & 1))
     lcd_print(ftostr31ns(current_position[X_AXIS]));
   else
     lcd_printPGM(PSTR("---"));
   u8g.drawPixel(49, XYZ_BASELINE - 5);
   u8g.drawPixel(49, XYZ_BASELINE - 3);
   u8g.setPrintPos(51, XYZ_BASELINE);
-  if (axis_known_position[Y_AXIS] || (blink & 1))
+  if (axis_homed[Y_AXIS] || (blink & 1))
     lcd_print(ftostr31ns(current_position[Y_AXIS]));
   else
     lcd_printPGM(PSTR("---"));
   u8g.drawPixel(89, XYZ_BASELINE - 5);
   u8g.drawPixel(89, XYZ_BASELINE - 3);
   u8g.setPrintPos(91, XYZ_BASELINE);
-  if (axis_known_position[Z_AXIS] || (blink & 1))
+  if (axis_homed[Z_AXIS] || (blink & 1))
     lcd_print(ftostr32sp(current_position[Z_AXIS]));
   else
     lcd_printPGM(PSTR("---.--"));

Marlin/ultralcd_implementation_hitachi_HD44780.h

       #else
 
         lcd.print('X');
-        if (axis_known_position[X_AXIS] || (blink & 1))
+        if (axis_homed[X_AXIS] || (blink & 1))
           lcd.print(ftostr4sign(current_position[X_AXIS]));
         else
           lcd_printPGM(PSTR(" ---"));
 
         lcd_printPGM(PSTR("  Y"));
-        if (axis_known_position[Y_AXIS] || (blink & 1))
+        if (axis_homed[Y_AXIS] || (blink & 1))
           lcd.print(ftostr4sign(current_position[Y_AXIS]));
         else
           lcd_printPGM(PSTR(" ---"));
 
     lcd.setCursor(LCD_WIDTH - 8, 1);
     lcd_printPGM(PSTR("Z "));
-    if (axis_known_position[Z_AXIS] || (blink & 1))
+    if (axis_homed[Z_AXIS] || (blink & 1))
       lcd.print(ftostr32sp(current_position[Z_AXIS] + 0.00001));
     else
       lcd_printPGM(PSTR("---.--"));