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.

serial.c

@@ -97,6 +97,7 @@ void serial_write(uint8_t data) {
 
   // Wait until there is space in the buffer
   while (next_head == serial_tx_buffer_tail) { 
+    // TODO: Restructure st_prep_buffer() calls to be executed here during a long print.    
     if (sys.execute & EXEC_RESET) { return; } // Only check for abort to avoid an endless loop.
   }