Refactor signing backend (#690)

Cleaned up the AES driver and removed some cppcheck isues
in the process.

examples/SecureActuator/SecureActuator.ino

@@ -121,4 +121,4 @@ void receive(const MyMessage &message)
 		Serial.println(message.getBool());
 	}
 }
-/** @}*/
+/** @}*/

examples/SecurityPersonalizer/sha204_library.cpp

@@ -261,7 +261,6 @@ uint8_t atsha204Class::sha204p_resync(uint8_t size, uint8_t *response)
 
 uint8_t atsha204Class::sha204p_receive_response(uint8_t size, uint8_t *response)
 {
-	uint8_t count_byte;
 	uint8_t i;
 	uint8_t ret_code;
 
@@ -273,6 +272,7 @@ uint8_t atsha204Class::sha204p_receive_response(uint8_t size, uint8_t *response)
 
 	ret_code = swi_receive_bytes(size, response);
 	if (ret_code == SWI_FUNCTION_RETCODE_SUCCESS || ret_code == SWI_FUNCTION_RETCODE_RX_FAIL) {
+		uint8_t count_byte;
 		count_byte = response[SHA204_BUFFER_POS_COUNT];
 		if ((count_byte < SHA204_RSP_SIZE_MIN) || (count_byte > size)) {
 			return SHA204_INVALID_SIZE;

examples/SecurityPersonalizer/sha204_library.h

@@ -326,7 +326,7 @@ public:
 	 *
 	 * @param[in]  pin   The pin to use for communication
 	 */
-	atsha204Class(uint8_t pin);
+	explicit atsha204Class(uint8_t pin);
 
 	/**
 	 * @brief      Wake up device