Add yield() for Teensy 2.0

teensy/core_pins.h

 }
 #endif
 
+void yield(void);
 
 extern void delay(uint32_t);
 

teensy/yield.cpp

+/* Teensyduino Core Library
+ * http://www.pjrc.com/teensy/
+ * Copyright (c) 2014 PJRC.COM, LLC.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * 1. The above copyright notice and this permission notice shall be 
+ * included in all copies or substantial portions of the Software.
+ *
+ * 2. If the Software is incorporated into a build system that allows 
+ * selection among a list of target devices, then similar target
+ * devices manufactured by PJRC.COM must be included in the list of
+ * target devices and selectable in the same manner.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "core_pins.h"
+//#include "HardwareSerial.h"
+
+void yield(void) __attribute__ ((weak));
+void yield(void)
+{
+	//static uint8_t running=0;
+
+	//if (running) return; // TODO: does this need to be atomic?
+	//running = 1;
+	//if (Serial.available()) serialEvent();
+	//if (Serial1.available()) serialEvent1();
+	//if (Serial2.available()) serialEvent2();
+	//if (Serial3.available()) serialEvent3();
+	//running = 0;
+};