Introducing the STM USB driver stack

STM32CubeMX was used as a starting point to generate a USB device driver stack with CDC support. Here's an overview of the significant USB source files and where they reside, relative to the root of the repository: https://github.com/PacktPublishing/Hands-On-RTOS-with-Microcontrollers/tree/master/

1. Low-level HAL USB files:

DRIVERS\STM32F7XX_HAL_DRIVER\
|--Inc
|    |stm32f7xx_ll_usb.h
|--Src
     |stm32f7xx_ll_usb.c

The stm32f7xx_ll_usb.c/h files are the lowest level files, which provide access to the USB hardware peripherals. These files are used by the STM-supplied USB driver stack middleware.

2. STM USB device stack:

MIDDLEWARE\ST\
|--STM32_USB_Device_Library
    |----Class
    | |----CDC
    |     |----Inc
    |         | usbd_cdc.h
    | ----Src
    | usbd_cdc.c
    |----Core
        |----Inc
        |    usbd_core.h
        |     usbd_ctlreq.h
 ...