We *should* use the persistent reference API for our netdev reference,

but there's so much else missing that it's not worth doing right now.

src/interface/pxe/pxe.c

 #include "dev.h"
 #include "pxe.h"
 
+#warning "pxe_netdev should hold a persistent reference to the net device"
 struct net_device *pxe_netdev = NULL;
 
 #if 0