network/netdev: introduce .is_ready_to_create() entry in netdev vtable

src/network/netdev/netdev.c

@@ -628,6 +628,9 @@ static bool netdev_is_ready_to_create(NetDev *netdev, Link *link) {
         if (link->set_link_messages > 0)
                 return false;
 
+        if (NETDEV_VTABLE(netdev)->is_ready_to_create)
+                return NETDEV_VTABLE(netdev)->is_ready_to_create(netdev, link);
+
         return true;
 }
 

src/network/netdev/netdev.h

@@ -153,6 +153,9 @@ typedef struct NetDevVTable {
         /* specifies if netdev is independent, or a master device or a stacked device */
         NetDevCreateType create_type;
 
+        /* This is used for stacked netdev. Return true when the underlying link is ready. */
+        int (*is_ready_to_create)(NetDev *netdev, Link *link);
+
         /* create netdev, if not done via rtnl */
         int (*create)(NetDev *netdev);