[cachedhcp] Automatically open network device matching cached DHCPACK

It is unintuitive to have to include an "ifopen" at the start of an
autoexec.ipxe script.  Provide a mechanism for upper-layer drivers to
mark a network device to be opened automatically upon registration,
and do so for the device to which the cached DHCPACK is applied.

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

src/include/ipxe/netdevice.h

@@ -461,6 +461,9 @@ struct net_device {
 /** Network device must be polled even when closed */
 #define NETDEV_INSOMNIAC 0x0040
 
+/** Network device should be opened automatically */
+#define NETDEV_AUTO_OPEN 0x0080
+
 /** Link-layer protocol table */
 #define LL_PROTOCOLS __table ( struct ll_protocol, "ll_protocols" )