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                                                    */