Rename configuration variables controling Python features.

Now of the form MICROPY_PY_*.  See issue #35.
diff --git a/py/objnamedtuple.c b/py/objnamedtuple.c
index a55dd64..6f228a8 100644
--- a/py/objnamedtuple.c
+++ b/py/objnamedtuple.c
@@ -34,7 +34,7 @@
 #include "obj.h"
 #include "objtuple.h"
 
-#if MICROPY_ENABLE_MOD_COLLECTIONS
+#if MICROPY_PY_COLLECTIONS
 
 typedef struct _mp_obj_namedtuple_type_t {
     mp_obj_type_t base;
@@ -174,4 +174,4 @@
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_namedtuple_obj, new_namedtuple_type);
 
-#endif // MICROPY_ENABLE_MOD_COLLECTIONS
+#endif // MICROPY_PY_COLLECTIONS