Use bool instead of boolean in drivers (#888)

In the Arduino IDE, boolean is an alias for bool.
The Arduino IDE might deprecate boolean in the future. See
https://github.com/arduino/Arduino/issues/4673 for details.

Command:
find drivers -type f | xargs sed -i 's/boolean/bool/g'

drivers/pubsubclient/CHANGES.txt

@@ -31,7 +31,7 @@
 1.8
     * KeepAlive interval is configurable in PubSubClient.h
     * Maximum packet size is configurable in PubSubClient.h
-    * API change: Return boolean rather than int from various functions
+    * API change: Return bool rather than int from various functions
     * API change: Length parameter in message callback changed
        from int to unsigned int
     * Various internal tidy-ups around types