usb: make usb_disconnect() weak

usb_disconnect() is board/platform specific for chips that don't have
built-in handling for this.  Allow the application to provide its own
version of usb_disconnect() to handle this.

See https://github.com/libopencm3/libopencm3/pull/924

include/libopencm3/usb/usbd.h

@@ -160,7 +160,12 @@ extern void usbd_register_set_altsetting_callback(usbd_device *usbd_dev,
 /* Functions to be provided by the hardware abstraction layer */
 extern void usbd_poll(usbd_device *usbd_dev);
 
-/** Disconnect, if supported by the driver */
+/** Disconnect, if supported by the driver
+ *
+ * This function is implemented as weak function and can be replaced by an
+ * application specific version to handle chips that don't have built-in
+ * handling for this (e.g. STM32F1.)
+ */
 extern void usbd_disconnect(usbd_device *usbd_dev, bool disconnected);
 
 /** Setup an endpoint

lib/usb/usb.c

@@ -112,7 +112,8 @@ void usbd_poll(usbd_device *usbd_dev)
 	usbd_dev->driver->poll(usbd_dev);
 }
 
-void usbd_disconnect(usbd_device *usbd_dev, bool disconnected)
+__attribute__((weak)) void usbd_disconnect(usbd_device *usbd_dev,
+					   bool disconnected)
 {
 	/* not all drivers support disconnection */
 	if (usbd_dev->driver->disconnect) {