net/dhcp: Actually send out DHCPv4 DISCOVER and REQUEST messages

Even though we were parsing some DHCP options sent by the server, so far
we are only using the BOOTP 2-way handshake, even when talking to a DHCP
server.

Change this by actually sending out DHCP DISCOVER packets instead of the
generic (mostly empty) BOOTP BOOTREQUEST packets.

A pure BOOTP server would ignore the extra DHCP options in the DISCOVER
packet and would just reply with a BOOTREPLY packet, which we also
handle in the code.

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

include/grub/net.h

@@ -462,9 +462,11 @@ enum
     GRUB_NET_BOOTP_DOMAIN = 0x0f,
     GRUB_NET_BOOTP_ROOT_PATH = 0x11,
     GRUB_NET_BOOTP_EXTENSIONS_PATH = 0x12,
+    GRUB_NET_DHCP_REQUESTED_IP_ADDRESS = 50,
     GRUB_NET_DHCP_OVERLOAD = 52,
     GRUB_NET_DHCP_MESSAGE_TYPE = 53,
     GRUB_NET_DHCP_SERVER_IDENTIFIER = 54,
+    GRUB_NET_DHCP_PARAMETER_REQUEST_LIST = 55,
     GRUB_NET_DHCP_TFTP_SERVER_NAME = 66,
     GRUB_NET_DHCP_BOOTFILE_NAME = 67,
     GRUB_NET_BOOTP_END = 0xff