package/kmod: add compression support for the host variant

package/kmod: add compression support for the host variant

The kernel may install compressed modules. At the end of the build, we
then run depmod, to generate modules.dep and a few assorted files, so
that loading modules works properly on the target (loading by alias,
loading dependencies...)

However, depmod needs support for compressed modules, or it would
generate empty modules.dep et al.

Uconditionally adding support for gz and xz, and the required deps,
was deemed too much of a burden [0], so we add options to enable
either or both compression.

[0] http://lists.busybox.net/pipermail/buildroot/2018-April/218410.html

Yegor Yefremov <yegorslists@googlemail.com>
Cc: Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Lucian Buga <lucianbuga@gmail.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>