net/dhcp: Introduce per-interface timeout

Currently we have a global timeout for all network cards in the BOOTP/DHCP
discovery process.

Make this timeout a per-interface one, so better accommodate the upcoming
4-way DHCP handshake and to also cover the lease time limit a DHCP offer
will come with.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/net.h

@@ -292,6 +292,8 @@ struct grub_net_network_level_interface
   struct grub_net_bootp_packet *dhcp_ack;
   grub_size_t dhcp_acklen;
   grub_uint16_t vlantag;
+  unsigned dhcp_tmo_left; /* DHCPv4 running retransmission timeout */
+  unsigned dhcp_tmo;      /* DHCPv4 current retransmission timeout */
   void *data;
 };