kernel: Fix running kernels *with* modules

Pull request #38470 added support for running/building kernels without
modules. This got merged in 38e04bbf

 but
unfortunately while this works perfectly on kernels without modules it
also makes sure that *every* kernel gets no modules.

So all of our VM tests fail since that merge with something like this:

machine# loading module loop...
machine# modprobe: FATAL: Module loop not found in directory /lib/modules/4.14.33
machine# loading module vfat...
machine# modprobe: FATAL: Module vfat not found in directory /lib/modules/4.14.33
machine# loading module nls_cp437...
machine# modprobe: FATAL: Module nls_cp437 not found in directory /lib/modules/4.14.33
machine# loading module nls_iso8859-1...
machine# modprobe: FATAL: Module nls_iso8859-1 not found in directory /lib/modules/4.14.33
machine# loading module fuse...
machine# modprobe: FATAL: Module fuse not found in directory /lib/modules/4.14.33
machine# loading module dm_mod...
machine# modprobe: FATAL: Module dm_mod not found in directory /lib/modules/4.14.33

I shortly tested this against the "misc" VM test and the test is working
again.

In the long term (and I currently don't have time for this) it would be
better to also have a VM test which tests a kernel without modules.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @roberth, @7c6f434c