Merge pull request #6404 from c1728p9/usb_updates

Update USBCDC and USBSerial

Author: cmonr

.travis.yml

@@ -111,7 +111,7 @@ matrix:
         - python tools/make.py -t GCC_ARM -m K64F --source=. --build=BUILD/K64F/GCC_ARM -j0
         # Check that example compiles without rtos
         - sed -n '/``` cpp/,/```/{/```$/Q;/```/d;p;}' $EVENTS/README.md > main.cpp
-        - rm -r rtos features/cellular features/netsocket features/frameworks BUILD
+        - rm -r rtos usb features/cellular features/netsocket features/frameworks BUILD
         - python tools/make.py -t GCC_ARM -m DISCO_F401VC --source=. --build=BUILD/DISCO_F401VC/GCC_ARM -j0
         # Run local equeue tests
         - make -C $EVENTS/equeue test