Added Print.availableForWrite()

Fix #1170

Signed-off-by: Frederic Pillon <[email protected]>

Author: fpistm

cores/arduino/Print.h

@@ -68,6 +68,10 @@ class Print {
       return write((const uint8_t *)buffer, size);
     }
 
+    // default to zero, meaning "a single write may block"
+    // should be overriden by subclasses with buffering
+    virtual int availableForWrite() { return 0; }
+
     size_t print(const __FlashStringHelper *);
     size_t print(const String &);
     size_t print(const char[]);