updsted the USB to persuade it to build for the Dragonfly platfom

Fork of USBDevice by mbed official

Revision:
70:2c525a50f1b6
Parent:
44:d2638fcd76d9
--- a/USBSerial/USBCDC.cpp	Fri Nov 11 17:59:00 2016 +0000
+++ b/USBSerial/USBCDC.cpp	Thu Jul 20 10:14:36 2017 +0100
@@ -38,6 +38,10 @@
     USBDevice::connect(connect_blocking);
 }
 
+void USBCDC::USBCallback_busReset(void) {
+    terminal_connected = false;
+};
+
 bool USBCDC::USBCallback_request(void) {
     /* Called in ISR context */