[settings] Move user-class setting from dhcp.c to settings.c

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

src/core/settings.c

 	.type = &setting_type_int8,
 };
 
+/** DHCP user class setting */
+struct setting user_class_setting __setting ( SETTING_HOST_EXTRA ) = {
+	.name = "user-class",
+	.description = "DHCP user class",
+	.tag = DHCP_USER_CLASS_ID,
+	.type = &setting_type_string,
+};
+
 /******************************************************************************
  *
  * Built-in settings block

src/include/ipxe/settings.h

 extern struct setting next_server_setting __setting ( SETTING_BOOT );
 extern struct setting mac_setting __setting ( SETTING_NETDEV );
 extern struct setting busid_setting __setting ( SETTING_NETDEV );
+extern struct setting user_class_setting __setting ( SETTING_HOST_EXTRA );
 
 /**
  * Initialise a settings block

src/net/udp/dhcp.c

 	.type = &setting_type_ipv4,
 };
 
-/** DHCP user class setting */
-struct setting user_class_setting __setting ( SETTING_HOST_EXTRA ) = {
-	.name = "user-class",
-	.description = "DHCP user class",
-	.tag = DHCP_USER_CLASS_ID,
-	.type = &setting_type_string,
-};
-
 /**
  * Most recent DHCP transaction ID
  *