package/cryptodev-linux: use the kernel-module helper

The kernel module needs the version.h headers at build-time, but the
Makefile only generates it when the module is built with that Makefile,
because it has this rule:

    build: version.h
        make ${KERNEL_MAKE_OPTS} modules

So, we must explicitly build this header, which we do in a pre-build hook
to ensure it comes before the kernel-module's own hooks. This header needs
not be installed in staging.

However, the cryptodev.h header still needs to be installed in staging,
which we still do in a staging-install command.

The KERNEL_DIR and PREFIX variables are no longer needed, because we're
no longer calling the rules that needed them (build, above, and
modules_install, both now handled by the kernel-module infra).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Doug: add pre-build hook to build version.h]
Signed-off-by: Doug Kehn <rdkehn@yahoo.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>