Implement crude but functional CDC + MSC USB device.

stm/Makefile

stm/lib/usb_conf.h

stm/lib/usbd_cdc_conf.h

stm/lib/usbd_cdc_core.c

stm/lib/usbd_cdc_core.h

stm/lib/usbd_cdc_vcp.c

stm/lib/usbd_conf.h

stm/lib/usbd_desc.c

stm/lib/usbd_desc.h

stm/lib/usbd_msc_bot.c

stm/lib/usbd_msc_conf.h

stm/lib/usbd_msc_core.c

stm/lib/usbd_msc_core.h

stm/lib/usbd_msc_scsi.c

stm/lib/usbd_pyb_core.c

stm/lib/usbd_pyb_core.h

stm/lib/usbd_req.c

stm/lib/usbd_req.h

stm/lib/usbd_storage_msd.c

stm/main.c