USBSerial

For STM32duino, USBSerial is part of USBDevice;

replacing USBDevice with USBLibrarySTM32 loses USBSerial.
Attempts to construct a USBSerial user library from USBDevice source fails:
- STM32duino core does not "see" it
- Arduino 2 compiles fail;  seemingly different rules apply for core compilation

After creating Documents/Arduino/libraries/USBSerial/,
builds still fail for USBSerial.h: No such file or directory
with USB support (if available): other than "None".

Note that it reports: Using library USBSerial at version 1.0.0 in folder:
  C:\Users\bleke\Documents\Arduino\libraries\USBSerial...??

Next, tried moving USBSerial to packages/STMicroelectronics/hardware/stm32/2.11.0/libraries/,
then modified platform.local.txt to include it.

- builds fail with file not found for some header files.
- STM32duino has e.g. hardware/stm32/2.11.0/system/extras/prebuild.sh
  that forces USBDevice library for -DUSBD_USE_CDC
- instead of changing USBSerial_include_dir in platform.local.txt,
  try replacing libraries/USBDevice contents...