usb_cdc: Allow USB endpoint ids to be board specific

It's common for boards to have restrictions on the type of each
endpoint.  So, make it possible for the board to select the endpoint
ids for each endpoint.

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

src/generic/usb_cdc_ep.h

@@ -0,0 +1,11 @@
+#ifndef __GENERIC_USB_CDC_EP_H
+#define __GENERIC_USB_CDC_EP_H
+
+// Default USB endpoint ids
+enum {
+    USB_CDC_EP_ACM = 1,
+    USB_CDC_EP_BULK_OUT = 2,
+    USB_CDC_EP_BULK_IN = 3,
+};
+
+#endif // usb_cdc_ep.h