[Settings] Add named setting for next-server

It's probably easier to have a named setting for next-server, rather
than to explain to people why they need to pretend it is option 175.3.

src/core/settings.c

 		.tag = DHCP_HOST_NAME,
 		.type = &setting_type_string,
 	},
+	{
+		.name = "next-server",
+		.description = "TFTP server",
+		.tag = DHCP_EB_SIADDR,
+		.type = &setting_type_ipv4,
+	},
 	{
 		.name = "filename",
 		.description = "Boot filename",