This changes the practice for building kernel modules: now we expect
that the appropriate Kconfig symbols are set to =m in
config.kernel.config, and then use pkgs.kmodloader to create
a service that loads and unloads all the modules depended on by
a particular requirement.
Note that modules won't be installed on the target device just by
virue of having been built: only the modules that are referenced by a
kmodloader package will be in the closure.
An example may make this clearer: see modules/firewall/default.nix
in this commit.
Why?
If you have a compiled Linux kernel source tree and you change some
symbol from "is not set" to m and then run make modules, you cannot in
general expect that newly compiled module to work. This is because
there are places in the build of the main kernel where it looks to see
which modules _may_ be defined and uses that information to
accommodate them.
For example in an in-kernel build of
https://github.com/torvalds/linux/blob/master/net/netfilter/core.c#L689
some symbols are defined only if CONFIG_NF_CONNTRACK is set, meaning
this code won't work if we have it unset initially then try later to
enable it and build modules only. Or see
https://github.com/torvalds/linux/blob/master/include/linux/netdevice.h#L160
