[syslog] Add support for IPv6 syslog server

Note that IANA has not yet assigned a DHCPv6 option code for the
syslog server.  When a code is assigned, the definition of
DHCPV6_LOG_SERVERS should be updated.  Until then, an IPv6 address of
a syslog server can be configured manually using e.g.

  set syslog6 3ffe:302:11:2::8309

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/dhcpv6.h

@@ -152,6 +152,13 @@ struct dhcpv6_user_class_option {
 /** DHCPv6 domain search list option */
 #define DHCPV6_DOMAIN_LIST 24
 
+/** DHCPv6 syslog server option
+ *
+ * This option code has not yet been assigned by IANA.  Please update
+ * this definition once an option code has been assigned.
+ */
+#define DHCPV6_LOG_SERVERS 0xffffffffUL
+
 /**
  * Any DHCPv6 option
  *