It's useful to provide a 'make clean' target as a lot of scripts expect
to be able to run it. Add empty 'clean' target to make those scripts
happy.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Add support for 'make check', which performs several checks on the
WHENCE file contents and on the git repository contents.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>