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.c b/net/bootp.c
index c15472f..b9e3ccc 100644
--- a/net/bootp.c
+++ b/net/bootp.c
@@ -603,7 +603,7 @@
 	*cnt += 1;
 #endif
 	if (IS_ENABLED(CONFIG_BOOTP_PXE_DHCP_OPTION)) {
-		*e++ = 209;	/* PXELINUX Config File */
+		*e++ = DHCP_OPTION_PXE_CONFIG_FILE;	/* PXELINUX Config File */
 		*cnt += 1;
 	}
 	/* no options, so back up to avoid sending an empty request list */
@@ -922,7 +922,7 @@
 				net_boot_file_name[size] = 0;
 			}
 			break;
-		case 209:	/* PXELINUX Config File */
+		case DHCP_OPTION_PXE_CONFIG_FILE:	/* PXELINUX Config File */
 			if (IS_ENABLED(CONFIG_BOOTP_PXE_DHCP_OPTION)) {
 				/* In case it has already been allocated when get DHCP Offer packet,
 				 * free first to avoid memory leak.
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__ */