usb_cdc: Add support for detecting a USB level Arduino bootloader request

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/usb_cdc.h

@@ -21,6 +21,7 @@ int_fast8_t usb_send_ep0_progmem(const void *data, uint_fast8_t len);
 void usb_stall_ep0(void);
 void usb_set_address(uint_fast8_t addr);
 void usb_set_configure(void);
+void usb_request_bootloader(void);
 
 // usb_cdc.c
 void usb_notify_bulk_in(void);