Add progmem support for String messages (#875)

added variant of MyMessage::set and sendSketchInfo that accept a
string that's stored in PROGMEM.

This avoids the need to (temporarily) store strings in memory to get
them into a message.

core/MySensorsCore.h

@@ -145,6 +145,9 @@ bool present(const uint8_t sensorId, const uint8_t sensorType, const char *descr
  * @return true Returns true if message reached the first stop on its way to destination.
  */
 bool sendSketchInfo(const char *name, const char *version, const bool ack = false);
+#if !defined(__linux__)
+bool sendSketchInfo(const __FlashStringHelper *name, const __FlashStringHelper *version, const bool ack = false);
+#endif
 
 /**
 * Sends a message to gateway or one of the other nodes in the radio network