libbsd commands and userspace-header-gen.py

[Chris Johns]

Hello,

I see the user commands in libbsd use a generated header file created by
`userspace-header-gen.py` however it does not run on my host because I do not
have an `elftools` package installed. I cannot find any details on what needs to
be installed so this tool can be used. I get the impression it is required to
make a new command work.

Thanks
Chris