Call shutdown() from pxenv_undi_shutdown(). Probably not totally

correct, but it's enough to get RIS working.

src/interface/pxe/pxe_undi.c

 #include <gpxe/pci.h>
 #include <gpxe/isapnp.h>
 #include <gpxe/if_ether.h>
+#include <gpxe/shutdown.h>
 #include "pxe.h"
 
 /* PXENV_UNDI_STARTUP
 				   *undi_shutdown ) {
 	DBG ( "PXENV_UNDI_SHUTDOWN" );
 
+	shutdown();
+
 	undi_shutdown->Status = PXENV_STATUS_SUCCESS;
 	return PXENV_EXIT_SUCCESS;
 }