Add ArduinoHALog helpers, extend ArduinoHADefines, update mqtt usage doc

src/ArduinoHALog.cpp

@@ -0,0 +1,11 @@
+#include "ArduinoHALog.h"
+
+static ArduinoHALogSink* g_arduinoHALogSink = nullptr;
+
+void arduinoHASetLogSink(ArduinoHALogSink* sink) {
+    g_arduinoHALogSink = sink;
+}
+
+ArduinoHALogSink* arduinoHAGetLogSink() {
+    return g_arduinoHALogSink;
+}