Fixes for compiling for Mega

Make more consistent use of architecture #defines.

libraries/MySensors/utility/ATSHA204.cpp

@@ -1,4 +1,4 @@
-#if defined(__AVR__)
+#if defined(ARDUINO_ARCH_AVR)
 
 #include "Arduino.h"
 #include "ATSHA204.h"
@@ -575,4 +575,4 @@ uint8_t ATSHA204Class::sha204c_check_crc(uint8_t *response)
     ? SHA204_SUCCESS : SHA204_BAD_CRC;
 }
 
-#endif // defined(__AVR__)
+#endif // defined(ARDUINO_ARCH_AVR)

libraries/MySensors/utility/RFM69.h

@@ -43,8 +43,8 @@
 #elif defined(__AVR_ATmega644P__) || defined(__AVR_ATmega1284P__) || defined(__AVR_ATmega2560__)
   #define RF69_IRQ_PIN          2
   #define RF69_IRQ_NUM          2
-#elif defined(ESP8266)
-  // TODO !!
+#elif defined(ARDUINO_ARCH_ESP8266)
+  // TODO !! Untested code! Entering unknown territory...
   #define RF69_IRQ_PIN          2
   #define RF69_IRQ_NUM          0
 #endif