Add before() for initialisations before radio hogs SPI

Fix #293

libraries/MySensors/core/MySensorCore.cpp

@@ -64,6 +64,9 @@ void _begin() {
 	    hwInit();
 	#endif
 
+	// Call before() in sketch (if it exists)
+	if (before) before();
+
 	debug(PSTR("Starting " MY_NODE_TYPE " (" MY_CAPABILIIES ", " LIBRARY_VERSION ")\n"));
 
 	signerInit();

libraries/MySensors/core/MySensorCore.h

@@ -236,6 +236,7 @@ extern MyMessage _msgTmp;  // Buffer for temporary messages (acks and nonces amo
 void receive(const MyMessage &message)  __attribute__((weak));
 void receiveTime(unsigned long)  __attribute__((weak));
 void presentation()  __attribute__((weak));
+void before() __attribute__((weak));
 void setup() __attribute__((weak));
 void loop() __attribute__((weak));