STM32: USB full-speed device driver

STM32: USB full-speed device driver

Add the low-level driver for the STM32 USB controller.

TODO: implement an intermediate hardware-agnostic USB layer to register
low-level USB constroller drivers and high-level USB device drivers:
     _________________________________
    |      High-level USB layer       |
    | (usb-serial, mass-storage, ...) |
    +---------------------------------+
    |     Middle-level USB layer      |
    |  (hardware-agnostic interface)  |
    +---------------------------------+
    |       Low-level USB layer       |
    |    (hardware-specific driver)   |
    +---------------------------------+

At the moment the "Middle-level" USB layer is merged into the
"Low-level" USB layer.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4240 38d2e660-2303-0410-9eaa-f027e97ec537