core/pkg-kernel-module: ensure linux supports modules

When a package wants to build a kernel module, we should ensure that the
kernel does support modules.

This patch does it automatically for packages using the kernel-module
infrastructure.

Packages that do not use it will have to set it manually (to be done in
a followup patch).

Suggested-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Noé Rubinstein <noe.rubinstein@gmail.com>
Cc: Jan Viktorin <viktorin@rehivetech.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>