fix:Example2 does not compile for Swan due to a missing directive (#65)

Author: bsatrom

examples/Example2_PeriodicCommunications/Example2_PeriodicCommunications.ino

@@ -21,6 +21,10 @@
 #define buttonPin     6
 #define buttonPressedState  LOW
 #define ledPin        4
+#elif defined(ARDUINO_ARCH_STM32)
+#define buttonPin     USER_BTN
+#define buttonPressedState  LOW
+#define ledPin        LED_BUILTIN
 #else
 #error "please add a board definition for button and led"
 #define buttonPin     ?       // Change to any GPIO pin where there is an active-high button