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/SPIFlash/SPIFlash.cpp

@@ -95,7 +95,7 @@ void SPIFlash::unselect()
 }
 
 /// setup SPI, read device ID etc...
-boolean SPIFlash::initialize()
+bool SPIFlash::initialize()
 {
 #if defined(SPCR) && defined(SPSR)
 	_SPCR = SPCR;
@@ -180,7 +180,7 @@ void SPIFlash::readBytes(uint32_t addr, void* buf, uint16_t len)
 }
 
 /// Send a command to the flash chip, pass TRUE for isWrite when its a write command
-void SPIFlash::command(uint8_t cmd, boolean isWrite)
+void SPIFlash::command(uint8_t cmd, bool isWrite)
 {
 #if defined(__AVR_ATmega32U4__) // Arduino Leonardo, MoteinoLeo
 	DDRB |= B00000001;            // Make sure the SS pin (PB0 - used by RFM12B on MoteinoLeo R1) is set as output HIGH!
@@ -201,7 +201,7 @@ void SPIFlash::command(uint8_t cmd, boolean isWrite)
 }
 
 /// check if the chip is busy erasing/writing
-boolean SPIFlash::busy()
+bool SPIFlash::busy()
 {
 	/*
 	select();

drivers/SPIFlash/SPIFlash.h

@@ -117,8 +117,8 @@ class SPIFlash
 public:
 	static uint8_t UNIQUEID[8]; //!< Storage for unique identifier
 	SPIFlash(uint8_t slaveSelectPin, uint16_t jedecID=0); //!< Constructor
-	boolean initialize(); //!< setup SPI, read device ID etc...
-	void command(uint8_t cmd, boolean isWrite=
+	bool initialize(); //!< setup SPI, read device ID etc...
+	void command(uint8_t cmd, bool isWrite=
 	                 false); //!< Send a command to the flash chip, pass TRUE for isWrite when its a write command
 	uint8_t readStatus(); //!< return the STATUS register
 	uint8_t readByte(uint32_t addr); //!< read 1 byte from flash memory
@@ -126,7 +126,7 @@ public:
 	void writeByte(uint32_t addr, uint8_t byt); //!< Write 1 byte to flash memory
 	void writeBytes(uint32_t addr, const void* buf,
 	                uint16_t len); //!< write multiple bytes to flash memory (up to 64K), if define SPIFLASH_SST25TYPE is set AAI Word Programming will be used
-	boolean busy(); //!< check if the chip is busy erasing/writing
+	bool busy(); //!< check if the chip is busy erasing/writing
 	void chipErase(); //!< erase entire flash memory array
 	void blockErase4K(uint32_t address); //!< erase a 4Kbyte block
 	void blockErase32K(uint32_t address); //!< erase a 32Kbyte block