py: Clean up instantiation of dupterm object.

To comply with already established scheme for extmod's.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 08dd497..5e717ad 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -819,12 +819,6 @@
 #define MICROPY_PORT_ROOT_POINTERS
 #endif
 
-#if MICROPY_PY_OS_DUPTERM
-#define MP_ROOT_PTR_DUPTERM_OBJ mp_obj_t term_obj
-#else
-#define MP_ROOT_PTR_DUPTERM_OBJ
-#endif
-
 /*****************************************************************************/
 /* Miscellaneous settings                                                    */
 
diff --git a/py/mpstate.h b/py/mpstate.h
index 012b0ef..a990713 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -127,6 +127,11 @@
     MICROPY_PORT_ROOT_POINTERS
 
     // root pointers for extmod
+
+    #if MICROPY_PY_OS_DUPTERM
+    mp_obj_t term_obj;
+    #endif
+
     #if MICROPY_PY_LWIP_SLIP
     mp_obj_t lwip_slip_stream;
     #endif
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 483c753..3be6168 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -665,3 +665,7 @@
 Q(sync)
 Q(count)
 #endif
+
+#if MICROPY_PY_OS_DUPTERM
+Q(dupterm)
+#endif