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/Linux/Arduino.h

@@ -99,7 +99,6 @@ using std::min;
 using std::max;
 using std::abs;
 
-typedef bool boolean;
 typedef uint8_t byte;
 typedef string String;
 typedef char __FlashStringHelper;