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

@@ -259,6 +259,7 @@ extern float home_offset[3]; // axis[n].home_offset
 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];