USB: implement generic usb-serial device driver

USB: implement generic usb-serial device driver

A generic board that includes this module can be plugged to a USB host
system (e.g., Linux, Windows, ...) to be seen as a generic usb-serial
adapter device (e.g., /dev/ttyUSB0 in Linux).

This module export the following functions to communicate between the
micro and the host over the usb-serial channel:

  ssize_t usb_serial_read(void *buf, ssize_t size);
  ssize_t usb_serial_write(const void *buf, ssize_t size);

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4241 38d2e660-2303-0410-9eaa-f027e97ec537