py/mpconfig.h: Mention MICROPY_PY_BTREE config option.

pfalcon · pfalcon · commit 737bd9c3144a · 2016-07-02T14:57:42.000+03:00
However, as it requires linking with external libraries, it actually
should be ste on Makefile level.
diff --git a/py/mpconfig.h b/py/mpconfig.h
@@ -899,6 +899,10 @@ typedef double mp_float_t;
 #define MICROPY_PY_FRAMEBUF (0)
 #endif
 
+#ifndef MICROPY_PY_BTREE
+#define MICROPY_PY_BTREE (0)
+#endif
+
 /*****************************************************************************/
 /* Hooks for a port to add builtins                                          */
 
