pkg-kernel-module: die if kernel module support is disabled

Test the configuration of the kernel to see if loadable module support
is enabled, and error out otherwise. This makes build failures of
external kernel modules less confusing.

[Thomas: tweak the commit log.]

Signed-off-by: Noé Rubinstein <nrubinstein@aldebaran.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>