availableForWrite (#4664)

added availableForWrite to Print.h to make compatible with Arduino Print.h

cores/esp32/Print.h

@@ -73,6 +73,11 @@ public:
     }
 
     size_t printf(const char * format, ...)  __attribute__ ((format (printf, 2, 3)));
+
+    // add availableForWrite to make compatible with Arduino Print.h
+    // 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[]);