kbuild: deb-pkg: do not include empty hook directories

kbuild: deb-pkg: do not include empty hook directories

The linux-image package currently includes empty hook directories
(/etc/kernel/{pre,post}{inst,rm}.d/ by default).

These directories were perhaps intended as a fail-safe in case no
hook scripts exist there.

However, they are really unnecessary because the run-parts command is
already guarded by the following check:

    test -d ${debhookdir}/${script}.d && run-parts ...

The only difference is that the run-parts command either runs for empty
directories (resulting in a no-op) or is skipped entirely.

The maintainer scripts will succeed without these dummy directories.

The linux-image packages from the Debian kernel do not contain
/etc/kernel/*.d/, either.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>