Reproducible builds progress: reprotest in sandbox example

[Jim MacArthur <jim macarthur codethink co uk>]

I've posted a project at 
https://gitlab.com/jmacarthur/reprotest-in-buildstream which contains a 
proof-of-concept of reprotest working inside a BuildStream sandbox and 
being used to break the reproducibility of 'nano'. The README contains 
all the details. This isn't integrated with BuildStream at all at the 
moment; you need to modify individual elements to use reprotest as part 
of the build.

There are some ways we could proceed with reproducibility at the end of 
the document. We also had a discussion on this subject on #buildstream 
yesterday:

https://irclogs.baserock.org/buildstream/%23buildstream.2018-02-13.log.html#t2018-02-13T16:21:57

Any comments are welcome.