[tftp] Allow builds without TFTP support

Allow TFTP to be configured out by moving the next-server setting
definition (which is used by autoboot.c) from tftp.c to settings.c.

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

src/core/settings.c

 	.type = &setting_type_string,
 };
 
+/** TFTP server setting */
+struct setting next_server_setting __setting ( SETTING_BOOT ) = {
+	.name = "next-server",
+	.description = "TFTP server",
+	.tag = DHCP_EB_SIADDR,
+	.type = &setting_type_ipv4,
+};
+
 /** Filename setting */
 struct setting filename_setting __setting ( SETTING_BOOT ) = {
 	.name = "filename",

src/net/udp/tftp.c

  ******************************************************************************
  */
 
-/** TFTP server setting */
-struct setting next_server_setting __setting ( SETTING_BOOT ) = {
-	.name = "next-server",
-	.description = "TFTP server",
-	.tag = DHCP_EB_SIADDR,
-	.type = &setting_type_ipv4,
-};
-
 /**
  * Apply TFTP configuration settings
  *