PFW-1362 temporary debug messages

Firmware/SpoolJoin.cpp

@@ -61,9 +61,14 @@ uint8_t SpoolJoin::isSpoolJoinEnabled()
 
 uint8_t SpoolJoin::nextSlot()
 {
+    SERIAL_ECHOPGM("SpoolJoin: Changing slot from ");
+    SERIAL_ECHO((int)currentMMUSlot);
     if (currentMMUSlot == 4) currentMMUSlot = 0;
     else currentMMUSlot++;
 
+    SERIAL_ECHOPGM(" to ");
+    SERIAL_ECHOLN((int)currentMMUSlot);
+
     return currentMMUSlot;
 }