add yield(default weak) (#70)

cores/arduino/Arduino.h

@@ -43,7 +43,7 @@ extern "C" {
 extern void setup(void) ;
 extern void loop(void) ;
 
-// void yield(void)
+void yield(void);
 
 #ifdef __cplusplus
 } // extern "C"

cores/arduino/hooks.c

@@ -0,0 +1,52 @@
+/*
+  Copyright (c) 2012 Arduino.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the GNU Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#include <stdint.h>
+
+/**
+ * Empty yield() hook.
+ *
+ * This function is intended to be used by library writers to build
+ * libraries or sketches that supports cooperative threads.
+ *
+ * Its defined as a weak symbol and it can be redefined to implement a
+ * real cooperative scheduler.
+ */
+static void __empty()
+{
+  // Empty
+}
+void yield(void) __attribute__((weak, alias("__empty")));
+
+#ifdef DTR_TOGGLING_SEQ
+/**
+ * Empty dtr_toggling() hook.
+ *
+ * This function is intended to be used by library writers to build
+ * libraries or sketches that require DTR toggling feature.
+ *
+ * Its defined as a weak symbol and it can be redefined to implement
+ * task to achieve in this case.
+ */
+static void __empty_dtr_toggling(uint8_t *buf, uint32_t *len)
+{
+  (void)buf;
+  (void)len;
+}
+void dtr_togglingHook(uint8_t *buf, uint32_t *len) __attribute__((weak, alias("__empty_dtr_toggling")));
+#endif