net: Enhancements for dhcp option 209

- Enable option 209 by default
- Set pxelinux_configfile to NULL to avoid potential double free
- change hardcoded 209 to a define

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
diff --git a/net/bootp.h b/net/bootp.h
index 4e32b19..24b32c7 100644
--- a/net/bootp.h
+++ b/net/bootp.h
@@ -91,6 +91,8 @@
 #define DHCP_NAK      6
 #define DHCP_RELEASE  7
 
+#define DHCP_OPTION_PXE_CONFIG_FILE	209	/* "ConfigFile" option according to rfc5071 */
+
 /**********************************************************************/
 
 #endif /* __BOOTP_H__ */