Add mTLS instructions

Change-Id: I4ac2896fa8d46418dbb08df9d9c08466fb51fdad
diff --git a/chain.ipxe b/chain.ipxe
index fa49bb3..3d2b840 100644
--- a/chain.ipxe
+++ b/chain.ipxe
@@ -6,4 +6,4 @@
 dhcp
 
 # chainload netboot.ipxe, or drop to shell if it doesn't exist
-chain http://boot.tucson.onlab.us/boot.ipxe || shell
+chain https://boot.tucson.onlab.us:2443/boot.ipxe || shell