fix small change

lib/usart.c

@@ -49,7 +49,8 @@ static volatile uint8_t rx_used = 0; /**< how much data has been received and no
 static uint8_t tx_buffer[USART_BUFFER] = {0}; /**< ring buffer for data to transmit */
 static volatile uint8_t tx_i = 0; /**< current position if transmitted data */
 static volatile uint8_t tx_used = 0; /**< how much data needs to be transmitted */
-volatile bool usart_received = 0; // same as rx_used, but since the user can write this variable we don't rely on it
+
+volatile bool usart_received = false;
 
 void usart_setup(void)
 {