kbuild: append -dirty for updated but uncommited changes

Compare the working copy with the last commit, instead of the index.

Signed-off-by: Uwe Zeisberger <zeisberg@informatik.uni-freiburg.de>
Acked-by: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>