systemd: fix path for kmod in service files

The path for kmod used in kmod-static-nodes.service is the one found by
the ./configure script, which ends up being the path to the host kmod
program.

This patch forces the path to the target kmod program.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>