[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