[dhcp] Assume PXE options are in DHCPOFFER only if boot menu is included

IBM's Tivoli Provisioning Manager for OS Deployment, when acting as a
ProxyDHCP server, sends an initial offer with a vendor class of "PXEClient"
and vendor-encapsulated options that have nothing to do with PXE. To
differentiate between this case and the case of a ProxyDHCP server that
sends all PXE options in its initial offer, modify gPXE to check for
the presence of an encapsulated PXE boot menu option (43.9) instead of
simply checking for the existence of any encapsulated options at all.
This is the same check used by the Intel vendor PXE ROM.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/net/udp/dhcp.c

 	has_pxeclient = ( ( vci_len >= ( int ) sizeof ( vci ) ) &&
 			  ( strncmp ( "PXEClient", vci, sizeof (vci) ) == 0 ));
 
-	/* Identify presence of vendor-specific options */
-	pxeopts_len = dhcppkt_fetch ( dhcppkt, DHCP_VENDOR_ENCAP, NULL, 0 );
+	/* Identify presence of PXE-specific options */
+	pxeopts_len = dhcppkt_fetch ( dhcppkt, DHCP_PXE_BOOT_MENU, NULL, 0 );
 	has_pxeopts = ( pxeopts_len >= 0 );
 	if ( has_pxeclient )
 		DBGC ( dhcp, "%s", ( has_pxeopts ? " pxe" : " proxy" ) );