Use descriptive constants instead of numeric literals

Marlin/Marlin_main.cpp

 
 #else // NOT DELTA
 
-      home_all_axis = !((code_seen(axis_codes[0])) || (code_seen(axis_codes[1])) || (code_seen(axis_codes[2])));
+      home_all_axis = !((code_seen(axis_codes[X_AXIS])) || (code_seen(axis_codes[Y_AXIS])) || (code_seen(axis_codes[Z_AXIS])));
 
       #if Z_HOME_DIR > 0                      // If homing away from BED do Z first
       if((home_all_axis) || (code_seen(axis_codes[Z_AXIS]))) {