linux: restore DEPMOD usage

The linux.mk rewrite lost the DEPMOD setting while installing modules
which means depending on host-module-init-tools has been useless.
Instead, the build system has been executing /sbin/depmod.

While we're here, drop the INSTALL_MOD_PATH since LINUX26_MAKE_FLAGS
already contains it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>