include: kernel.mk: simplify module autoloading

Let the generic postinstall script invoke "kmodloader" when the just
installed package contains any /etc/module.d/ entries.

This allows us to skip the explicit "insert_module()" calls in the
package postinstall.

Due to the removed insert_module calls we do not need to assemble a
complete list of modules per package anymore, which allows for vast
simplification of the package generation code.

While we're at it, also support specifying default parameters for
modules using either the MODPARAM or MODPARAM.modulename variables
in KernelPackage.

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