Implement crude but functional CDC + MSC USB device.

stm/usb.c

-#include "usbd_cdc_core.h"
-#include "usbd_msc_core.h"
+#include "usb_core.h"
+#include "usbd_core.h"
+#include "usbd_pyb_core.h"
 #include "usbd_usr.h"
 #include "usbd_desc.h"
 
@@ -15,6 +16,6 @@ void usb_vcp_send(const char* str, int len) {
     //APP_FOPS.pIf_DataTx(str, len);
 }
 
-void usb_msc_init() {
-    USBD_Init(&USB_OTG_dev, USB_OTG_FS_CORE_ID, &USR_desc, &USBD_MSC_cb, &USR_cb);
+void usb_init() {
+    USBD_Init(&USB_OTG_dev, USB_OTG_FS_CORE_ID, &USR_desc, &USBD_PYB_cb, &USR_cb);
 }