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/MyMessage.h

@@ -325,6 +325,9 @@ public:
 	// Setters for payload
 	MyMessage& set(void* payload, uint8_t length);
 	MyMessage& set(const char* value);
+#if !defined(__linux__)
+	MyMessage& set(const __FlashStringHelper* value);
+#endif
 	MyMessage& set(float value, uint8_t decimals);
 	MyMessage& set(bool value);
 	MyMessage& set(uint8_t value);