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/cmd/Kconfig b/cmd/Kconfig
index c182d73..54c4ab8 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1898,6 +1898,7 @@
 
 config BOOTP_PXE_DHCP_OPTION
 	bool "Request & store 'pxe_configfile' from BOOTP/DHCP server"
+	default y
 	depends on BOOTP_PXE
 
 config BOOTP_VCI_STRING
diff --git a/cmd/pxe.c b/cmd/pxe.c
index 21134eb..9404f44 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -65,6 +65,8 @@
 	int ret = get_pxe_file(ctx, pxelinux_configfile, pxefile_addr_r);
 
 	free(pxelinux_configfile);
+	/* set to NULL to avoid double-free if DHCP is tried again */
+	pxelinux_configfile = NULL;
 
 	return ret;
 }