G28/30 bug fix. Block '$$' during cycle. SO2 defaults.

- A G28/30 bug would cause it to move to machine coordinate [0,0,0] if
no axis words were sent. It was a typo in the new g-code parser. Fixed
and slightly re-written to be more consistent with the program flow.

- Updated the ShapeOko2 defaults based on testing on the real machine.
A little conservative, but it might change again after some more
testing.

- Now blocks ‘$$’ command during a motion, because the printout takes
too long and can starve the segment buffer.

system.c

@@ -88,6 +88,7 @@ uint8_t system_execute_line(char *line)
     case 0 : report_grbl_help(); break;
     case '$' : // Prints Grbl settings
       if ( line[++char_counter] != 0 ) { return(STATUS_INVALID_STATEMENT); }
+      if ( sys.state & (STATE_CYCLE | STATE_HOLD) ) { return(STATUS_IDLE_ERROR); } // Block during cycle. Takes too long to print.
       else { report_grbl_settings(); }
       break;
     case 'G' : // Prints gcode parser state