kbuild, deb-pkg: allow alternative hook scripts directory in .deb packages

Hook scripts in the default directory /etc/kernel are also executed by
official Debian kernel packages as well as kernel packages created using
make-kpkg. Allow to specify an alternative hook scripts directory by
exporting the environment variable KDEB_HOOKDIR.

Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: maximilian attems <max@stro.at>
Cc: Andres Salomon <dilinger@debian.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>