usb_cdcacm: add flush method to wait for empty buffer

lib/usb_cdcacm.c

@@ -479,7 +479,13 @@ void usb_cdcacm_putchar(char c)
 	usb_cdcacm_data_tx_cb(NULL, 0); // send data over USB when possible
 }
 
+void usb_cdcacm_flush(void)
+{
+	while (tx_used);
+}
+
 /** USB interrupt service routine called when data is received */
-void usb_lp_can_rx0_isr(void) {
+void usb_lp_can_rx0_isr(void)
+{
 	usbd_poll(usb_device);
 }

lib/usb_cdcacm.h

@@ -16,3 +16,6 @@ void usb_cdcacm_setup(void);
  *  @note blocks if transmit buffer is full, else puts in buffer and returns
  */
 void usb_cdcacm_putchar(char c);
+/** wait until all is transmitted (blocking)
+ */
+void usb_cdcacm_flush(void);