Add sketch code to reassemble a DHCP packet from our internal "everything

is a DHCP option" data structures.

We need this code in order to be able to return a DHCP packet to a PXE NBP
which reflects options from our multiple sources (e.g. NVS and DHCP
server).  This is expensive, but necessary.  Having paid this cost, we may
as well try to use the same code to generate our DHCP request packets,
since the process is similar.

src/include/gpxe/dhcp.h

@@ -118,6 +118,12 @@ struct dhcp_packet {
  */
 #define DHCP_PAD 0
 
+/** Minimum normal DHCP option */
+#define DHCP_MIN_OPTION 1
+
+/** Vendor encapsulated options */
+#define DHCP_VENDOR_ENCAP 43
+
 /** Option overloading
  *
  * The value of this option is the bitwise-OR of zero or more
@@ -131,6 +137,17 @@ struct dhcp_packet {
 /** The "sname" field is overloaded to contain extra DHCP options */
 #define DHCP_OPTION_OVERLOAD_SNAME 2
 
+/** DHCP message type */
+#define DHCP_MESSAGE_TYPE 53
+#define DHCPDISCOVER 1
+#define DHCPOFFER 2
+#define DHCPREQUEST 3
+#define DHCPDECLINE 4
+#define DHCPACK 5
+#define DHCPNAK 6
+#define DHCPRELEASE 7
+#define DHCPINFORM 8
+
 /** TFTP server name
  *
  * This option replaces the fixed "sname" field, when that field is
@@ -178,6 +195,9 @@ struct dhcp_packet {
  */
 #define DHCP_EB_SIADDR DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 3 )
 
+/** Maximum normal DHCP option */
+#define DHCP_MAX_OPTION 254
+
 /** End of options
  *
  * This tag does not have a length field; it is always only a single
@@ -260,6 +280,8 @@ find_dhcp_option ( struct dhcp_option_block *options, unsigned int tag );
 extern struct dhcp_option * find_global_dhcp_option ( unsigned int tag );
 extern void register_dhcp_options ( struct dhcp_option_block *options );
 extern void unregister_dhcp_options ( struct dhcp_option_block *options );
+extern void init_dhcp_options ( struct dhcp_option_block *options,
+				void *data, size_t max_len );
 extern struct dhcp_option_block * alloc_dhcp_options ( size_t max_len );
 extern void free_dhcp_options ( struct dhcp_option_block *options );
 extern struct dhcp_option *