[dhcp] Ensure message type is first DHCP option

Apparently, the DHCP relay logic on a Nortel 470-48T layer 2 switch
cares about the order of DHCP options.  Specifically, it requires
that the DHCP message type option be the first option present in the
DHCP packet.  We achieve this by having this option appear first in
our dhcp_request_options_data array, which pre-populates DHCP
requests.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/net/udp/dhcp.c

 
 /** Raw option data for options common to all DHCP requests */
 static uint8_t dhcp_request_options_data[] = {
+	DHCP_MESSAGE_TYPE, DHCP_BYTE ( 0 ),
 	DHCP_MAX_MESSAGE_SIZE,
 	DHCP_WORD ( ETH_MAX_MTU - 20 /* IP header */ - 8 /* UDP header */ ),
 	DHCP_CLIENT_ARCHITECTURE, DHCP_WORD ( 0 ),