Rename configuration variables controling Python features.

Now of the form MICROPY_PY_*.  See issue #35.
diff --git a/py/modio.c b/py/modio.c
index b1f0d88..e1761f7 100644
--- a/py/modio.c
+++ b/py/modio.c
@@ -30,7 +30,7 @@
 #include "obj.h"
 #include "builtin.h"
 
-#if MICROPY_ENABLE_MOD_IO
+#if MICROPY_PY_IO
 
 extern const mp_obj_type_t mp_type_fileio;
 extern const mp_obj_type_t mp_type_textio;
@@ -40,14 +40,14 @@
     // Note: mp_builtin_open_obj should be defined by port, it's not
     // part of the core.
     { MP_OBJ_NEW_QSTR(MP_QSTR_open), (mp_obj_t)&mp_builtin_open_obj },
-    #if MICROPY_MOD_IO_FILEIO
+    #if MICROPY_PY_IO_FILEIO
     { MP_OBJ_NEW_QSTR(MP_QSTR_FileIO), (mp_obj_t)&mp_type_fileio },
     #endif
     #if MICROPY_CPYTHON_COMPAT
     { MP_OBJ_NEW_QSTR(MP_QSTR_TextIOWrapper), (mp_obj_t)&mp_type_textio },
     #endif
     { MP_OBJ_NEW_QSTR(MP_QSTR_StringIO), (mp_obj_t)&mp_type_stringio },
-    #if MICROPY_IO_BYTESIO
+    #if MICROPY_PY_IO_BYTESIO
     { MP_OBJ_NEW_QSTR(MP_QSTR_BytesIO), (mp_obj_t)&mp_type_bytesio },
     #endif
 };