kmod: allow to build a host variant

Since module-init-tools is deprecated, we now want to use kmod instead
for the installation of modules during the kernel build process. In
order to do this, we need to be able to build a host variant of kmod,
which is want this patch allows to do.

Note that only the depmod tool is installed on the host, since that's
the only one likely to be used on the host in a cross-compilation
context.

[Peter: needs AUTORECONF for the host as well. Fix comment while we're at it]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>