Add printf for Teensy3

teensy3/Print.cpp

 	return write(buf, 2);
 }
 
+extern "C" {
+__attribute__((weak))
+int _write(int file, char *ptr, int len)
+{
+	((class Print *)file)->write((uint8_t *)ptr, len);
+	return 0;
+}
+}
+
+int Print::printf(const char *format, ...)
+{
+	va_list ap;
+	va_start(ap, format);
+	return vdprintf((int)this, format, ap);
+}
+
+int Print::printf(const __FlashStringHelper *format, ...)
+{
+	va_list ap;
+	va_start(ap, format);
+	return vdprintf((int)this, (const char *)format, ap);
+}
+
 
 size_t Print::printNumber(unsigned long n, uint8_t base, uint8_t sign)
 {

teensy3/Print.h

 
 #include <inttypes.h>
 #include <stdio.h> // for size_t - gives sprintf and other stuff to all sketches & libs
+#include <stdarg.h>
 #include "core_id.h"
 #include "WString.h"
 #include "Printable.h"
 	size_t println(const Printable &obj)		{ return obj.printTo(*this) + println(); }
 	int getWriteError() { return write_error; }
 	void clearWriteError() { setWriteError(0); }
-	size_t printNumber(unsigned long n, uint8_t base, uint8_t sign);
+	int printf(const char *format, ...);
+	int printf(const __FlashStringHelper *format, ...);
   protected:
 	void setWriteError(int err = 1) { write_error = err; }
   private:
 	char write_error;
 	size_t printFloat(double n, uint8_t digits);
+	size_t printNumber(unsigned long n, uint8_t base, uint8_t sign);
 };
 
 

teensy3/mk20dx128.c

 	return 0;
 }
 
+/*  moved to Print.cpp, to support Print::printf()
 __attribute__((weak)) 
 int _write(int file, char *ptr, int len)
 {
 	return 0;
 }
+*/
 
 __attribute__((weak)) 
 int _close(int fd)