feat: add Print::vprintf function

This does the same as printf, but (like the vprintf libc function)
accepts an already processed va_list to allow other vararg functions to
forward their argument lists to this function.

Author: matthijskooijman

Diff for: cores/arduino/Print.cpp

@@ -277,6 +277,17 @@ int Print::printf(const __FlashStringHelper *format, ...)
   return retval;
 }
 
+int Print::vprintf(const char *format, va_list ap)
+{
+  return vdprintf((int)this, format, ap);
+}
+
+int Print::vprintf(const __FlashStringHelper *format, va_list ap)
+{
+  return vdprintf((int)this, (const char*)format, ap);
+}
+
+
 // Private Methods /////////////////////////////////////////////////////////////
 
 size_t Print::printNumber(unsigned long n, uint8_t base)

Diff for: cores/arduino/Print.h

@@ -106,6 +106,8 @@ class Print {
 
     int printf(const char *format, ...);
     int printf(const __FlashStringHelper *format, ...);
+    int vprintf(const __FlashStringHelper *format, va_list ap);
+    int vprintf(const char *format, va_list ap);
 
     virtual void flush() { /* Empty implementation for backward compatibility */ }
 };