kpatch-build: DEBUG for create-diff-object

Add -d option to create-diff-object when DEBUG is set.  That way for
weird kpatch-build issues we can just tell people to use the -d flag and
then provide the build log.
fix review comment
Merge pull request #255 from jpoimboe/kpatch-build-debug

kpatch-build: DEBUG for create-diff-object