package-infra: add helper to build kernel modules

package-infra: add helper to build kernel modules

The Linux kernel offers a nice and easy-to-use infra to build
out-of-tree kernel modules.

Currently, we have quite a few packages that build kernel modules, and
most duplicate (or rewrite) the same code over-and-over again.

Introduce a new infrastructure that provides helpers to build kernel
modules, so packages do not have to duplicate/rewrite that.

The infrastructure, unlike any other package infra, is not standalone.
It needs another package infra to be used. This is so that packages that
provide both userland and kernel modules can be built easily. So, this
infra only defines post-build and post-install hooks, that will build
the kernel modules after the rest of the package.

We need to override PWD, because some packages will use it to find their
own includes (and other helper files). PWD is inherited from the
environment, so it gets whatever value it had when make was launched,
which happens to be Buildroot's own top source tree. So, we just force
PWD to the proper value, rather than cd-ing first.

Also, no host version is provided, since it does not make sense to build
kernel modules for the host.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>