bootp: ignore gateway_ip (relay) field.

From RFC1542:

   The 'giaddr' field is rather poorly named.  It exists to facilitate
   the transfer of BOOTREQUEST messages from a client, through BOOTP
   relay agents, to servers on different networks than the client.
   Similarly, it facilitates the delivery of BOOTREPLY messages from the
   servers, through BOOTP relay agents, back to the client.  In no case
   does it represent a general IP router to be used by the client.  A
   BOOTP client MUST set the 'giaddr' field to zero (0.0.0.0) in all
   BOOTREQUEST messages it generates.

   A BOOTP client MUST NOT interpret the 'giaddr' field of a BOOTREPLY
   message to be the IP address of an IP router.  A BOOTP client SHOULD
   completely ignore the contents of the 'giaddr' field in BOOTREPLY
   messages.

Leave code ifdef'd out for the time being in case we see regression.

Suggested by: Rink Springer <rink@rink.nu>
Closes: 43396

grub-core/net/bootp.c

@@ -157,6 +157,12 @@ grub_net_configure_by_dhcp_ack (const char *name,
   hwaddr.type = GRUB_NET_LINK_LEVEL_PROTOCOL_ETHERNET;
 
   inter = grub_net_add_addr (name, card, &addr, &hwaddr, flags);
+#if 0
+  /* This is likely based on misunderstanding. gateway_ip refers to
+     address of BOOTP relay and should not be used after BOOTP transaction
+     is complete.
+     See RFC1542, 3.4 Interpretation of the 'giaddr' field
+   */
   if (bp->gateway_ip)
     {
       grub_net_network_level_netaddress_t target;
@@ -178,6 +184,7 @@ grub_net_configure_by_dhcp_ack (const char *name,
       target.ipv4.masksize = 32;
       grub_net_add_route (name, target, inter);
     }
+#endif
 
   if (size > OFFSET_OF (boot_file, bp))
     grub_env_set_net_property (name, "boot_file", bp->boot_file,