Write out an iPXE script file for Packet.
The script uses relative URLs to refer to
the other PXE files and thus can be copied
along with the files to any server.
This is useful because it saves the creation
of an iPXE script for a release/channel on a
third-party service. For CI testing it is
also helpful because the script does not only
end up on the release server but also already
on the Google buckets, refering to unpublished
PXE payloads.
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