[pxe] Reopen network device if NBP exits

Attempt to restore the network device to the state it was in prior to
calling the NBP.  This simplifies the task of taking follow-up action
in an iPXE script.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/pxe_image.c

@@ -65,6 +65,7 @@ static int pxe_exec ( struct image *image ) {
 		       image );
 		return -ENODEV;
 	}
+	netdev_get ( netdev );
 
 	/* Activate PXE */
 	pxe_activate ( netdev );
@@ -81,6 +82,12 @@ static int pxe_exec ( struct image *image ) {
 	/* Deactivate PXE */
 	pxe_deactivate();
 
+	/* Try to reopen network device.  Ignore errors, since the NBP
+	 * may have called PXENV_STOP_UNDI.
+	 */
+	netdev_open ( netdev );
+	netdev_put ( netdev );
+
 	return rc;
 }