Generate local-vars.ipxe by default with examples

Adds the local-vars.ipxe file which can be updated
for changes.  Local vars is parsed on bootup if
the next-server and filename are both set on the
local dhcp server.

script/netbootxyz-overrides.yml

@@ -5,6 +5,7 @@ generate_disks_arm: true
 generate_disks_hybrid: true
 generate_disks_rpi: true
 generate_version_file: true
+generate_local_vars: false
 bootloader_multiple: true
 bootloader_disks:
   - "netboot.xyz"