kbuild: Do not write to builddir in modules_install

Let depmod.sh create a temporary directory in /tmp instead of writing to
the build directory as root. The mktemp utility should be available on
any recent system (and there is already scripts/gen_initramfs_list.sh
relying on it).

Reported-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>