[ntp] Define NTP server setting

Define the IPv4 NTP server setting to simplify the use of a
DHCP-provided NTP server in scripts, using e.g.

  #!ipxe
  dhcp
  ntp ${ntp}

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

src/include/ipxe/dhcp.h

@@ -86,6 +86,9 @@ struct dhcp_packet;
 /** Maximum transmission unit */
 #define DHCP_MTU 26
 
+/** NTP servers */
+#define DHCP_NTP_SERVERS 42
+
 /** Vendor encapsulated options */
 #define DHCP_VENDOR_ENCAP 43
 

src/net/udp/ntp.c

@@ -36,6 +36,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/timer.h>
 #include <ipxe/time.h>
 #include <ipxe/tcpip.h>
+#include <ipxe/dhcp.h>
+#include <ipxe/settings.h>
 #include <ipxe/ntp.h>
 
 /** @file
@@ -273,3 +275,11 @@ int start_ntp ( struct interface *job, const char *hostname ) {
  err_alloc:
 	return rc;
 }
+
+/** IPv4 NTP server setting */
+const struct setting ntp_setting __setting ( SETTING_IP4_EXTRA, ntp ) = {
+	.name = "ntp",
+	.description = "NTP server",
+	.tag = DHCP_NTP_SERVERS,
+	.type = &setting_type_ipv4,
+};