[settings] Move "domain" setting from dns.c to settings.c

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

src/core/settings.c

 	.type = &setting_type_string,
 };
 
+/** Domain name setting */
+struct setting domain_setting __setting ( SETTING_IPv4_EXTRA ) = {
+	.name = "domain",
+	.description = "DNS domain",
+	.tag = DHCP_DOMAIN_NAME,
+	.type = &setting_type_string,
+};
+
 /** TFTP server setting */
 struct setting next_server_setting __setting ( SETTING_BOOT ) = {
 	.name = "next-server",

src/include/ipxe/settings.h

 extern struct setting gateway_setting __setting ( SETTING_IPv4 );
 extern struct setting dns_setting __setting ( SETTING_IPv4_EXTRA );
 extern struct setting hostname_setting __setting ( SETTING_HOST );
+extern struct setting domain_setting __setting ( SETTING_IPv4_EXTRA );
 extern struct setting filename_setting __setting ( SETTING_BOOT );
 extern struct setting root_path_setting __setting ( SETTING_SANBOOT );
 extern struct setting username_setting __setting ( SETTING_AUTH );

src/net/udp/dns.c

 	.type = &setting_type_ipv4,
 };
 
-/** Domain name setting */
-struct setting domain_setting __setting ( SETTING_IPv4_EXTRA ) = {
-	.name = "domain",
-	.description = "DNS domain",
-	.tag = DHCP_DOMAIN_NAME,
-	.type = &setting_type_string,
-};
-
 /**
  * Apply DNS settings
  *