Add printf to print class (stm32duino#780)

Based on Adafruits printf implementation in
https://github.com/adafruit/ArduinoCore-samd

Author: MCUdude

cores/arduino/Print.cpp

@@ -196,6 +196,26 @@ size_t Print::println(const Printable &x)
   return n;
 }
 
+void Print::printf(const char *format, ...)
+{
+  char buf[PRINTF_BUFFER];
+  va_list ap;
+  va_start(ap, format);
+  vsnprintf(buf, sizeof(buf), format, ap);
+  write(buf);
+  va_end(ap);
+}
+
+void Print::printf(const __FlashStringHelper *format, ...)
+{
+  char buf[PRINTF_BUFFER];
+  va_list ap;
+  va_start(ap, format);
+  vsnprintf_P(buf, sizeof(buf), (const char *)format, ap);
+  write(buf);
+  va_end(ap);
+}
+
 // Private Methods /////////////////////////////////////////////////////////////
 
 size_t Print::printNumber(unsigned long n, uint8_t base)

cores/arduino/Print.h

@@ -22,6 +22,7 @@
 
 #include <inttypes.h>
 #include <stdio.h> // for size_t
+#include <stdarg.h> // for printf
 
 #include "WString.h"
 #include "Printable.h"
@@ -31,6 +32,10 @@
 #define OCT 8
 #define BIN 2
 
+#ifndef PRINTF_BUFFER
+#define PRINTF_BUFFER 80
+#endif
+
 // uncomment next line to support printing of 64 bit ints.
 #define SUPPORT_LONGLONG
 
@@ -103,6 +108,9 @@ class Print {
     void println(uint64_t, uint8_t = DEC);
     void print(uint64_t, uint8_t = DEC);
 #endif
+
+    void printf(const char *format, ...);
+    void printf(const __FlashStringHelper *format, ...);
 };
 
 #endif