Move debian/ directory generation out of builddeb to a new script,
mkdebian. The package build commands are kept in builddeb, which
is now an internal command called from debian/rules.

With these changes in place, we can now use dpkg-buildpackage from
deb-pkg and bindeb-pkg removing need for handrolled source/changes
generation.

This patch is based on the criticism of the current state of builddeb
discussed on:

https://patchwork.kernel.org/patch/9656403/Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>