openswitch: improve build time for kmod-only builds

openswitch: improve build time for kmod-only builds

When only kmod-openvswitch is enabled, which commonly happens during LEDE
release builds, there is no need to build the entire userland of openvswitch
as this is done at a later stage on systems dedicated to build only userspace
packages.

This change conditionalizes the dependency on python, which means that python
prerequisites are only compiled if the openvswitch-python package is enabled,
which allows us to eliminate the entire python dependency chain.

Furthermore, this change sets MAKE_PATH to the kernel module sub directory
if only kmod-openvswitch is enabled which causes the openvswitch build system
to only process sources related to the .ko files.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>