Allow ports to define statically builtin functions.
Convert unix open() to such.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 1eb2acf..6ff0692 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -107,6 +107,10 @@
#define MICROPY_PATH_MAX (512)
#endif
+// Additional builtin function definitions - see runtime.c:builtin_table for format.
+#ifndef MICROPY_EXTRA_BUILTINS
+#define MICROPY_EXTRA_BUILTINS
+#endif
/*****************************************************************************/
/* Miscellaneous settings */