Add "externally_visible" attribute for usbd_core entry point and drivers.

src/usbd_core.c

@@ -352,6 +352,6 @@ static void usbd_process_evt(usbd_device *dev, uint8_t evt, uint8_t ep) {
     if (dev->events[evt]) dev->events[evt](dev, evt, ep);
 }
 
-void usbd_poll(usbd_device *dev) {
+ __attribute__((externally_visible)) void usbd_poll(usbd_device *dev) {
     return dev->driver->poll(dev, usbd_process_evt);
 }

src/usbd_stm32f103_devfs.c

@@ -531,7 +531,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_devfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_devfs = {
     getinfo,
     enable,
     connect,

src/usbd_stm32f105_otgfs.c

@@ -465,7 +465,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_otgfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_otgfs = {
     getinfo,
     enable,
     connect,

src/usbd_stm32f429_otgfs.c

@@ -463,7 +463,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_otgfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_otgfs = {
     getinfo,
     enable,
     connect,

src/usbd_stm32l052_devfs.c

@@ -452,7 +452,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_devfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_devfs = {
     getinfo,
     enable,
     connect,

src/usbd_stm32l100_devfs.c

@@ -441,7 +441,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_devfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_devfs = {
     getinfo,
     enable,
     connect,

src/usbd_stm32l476_otgfs.c

@@ -480,7 +480,7 @@ uint16_t get_serialno_desc(void *buffer) {
     return 18;
 }
 
-const struct usbd_driver usbd_otgfs = {
+ __attribute__((externally_visible)) const struct usbd_driver usbd_otgfs = {
     getinfo,
     enable,
     connect,