add debug option to kpatch-build

When debugging kpatch-build failures it can be
beneficial to have the scratch files in /tmp that
kpatch-build was operating on.  These are
removed by default, as they can quickly fill /tmp.
However, for debugging reasons, the option should
exist to keep them around.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Merge pull request #95 from spartacus06/debug-option

add debug option to kpatch-build