Moves 'pylint' to its own target, adds a 'pep8' target, and makes
the 'check' target empty with 'check' and 'pep8' as
prerequisites.
This allows the user to run either pylint or pep8 separately or
together.
It also makes it possible to plumb a user-specified PYTHONPATH
into the 'pylint' target.
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To view this pull request on github, visit
https://github.com/rhinstaller/blivet/pull/272