[pxe] Construct all fake DHCP packets before starting PXE NBP

Commit edf74df ("[pxe] Always reconstruct packet for
PXENV_GET_CACHED_INFO") fixed the problems caused by returning stale
DHCP packets (e.g. from an earlier boot attempt using a different
network device), but broke interoperability with NBPs such as WDS
which may overwrite our cached (fake) DHCP packets and expect the
modified packets to be returned by a subsequent call to
PXENV_GET_CACHED_INFO.

Fix by constructing the fake DHCP packets immediately before
transferring control to a PXE NBP.  Calls to PXENV_GET_CACHED_INFO
will now never modify the cached packets.

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

src/arch/i386/image/pxe_image.c

@@ -78,6 +78,9 @@ static int pxe_exec ( struct image *image ) {
 	/* Activate PXE */
 	pxe_activate ( netdev );
 
+	/* Construct fake DHCP packets */
+	pxe_fake_cached_info();
+
 	/* Set PXE command line */
 	pxe_cmdline = image->cmdline;