network/netdev: introduce .iftype to netdev vtable

And disable .generate_mac flag for non-ether interfaces.

src/network/netdev/netdev.h

@@ -159,6 +159,9 @@ typedef struct NetDevVTable {
         /* verify that compulsory configuration options were specified */
         int (*config_verify)(NetDev *netdev, const char *filename);
 
+        /* expected iftype, e.g. ARPHRD_ETHER. */
+        uint16_t iftype;
+
         /* Generate MAC address when MACAddress= is not specified. */
         bool generate_mac;
 } NetDevVTable;