Add kmod repo integration

When the kmod packaging infrastructure was originally added the
dependency on the rpmfusion yum repositories was disabled.  This
was done at the time in favour of getting local builds working.

Now the time has come to conditionally re-enable that functionality
so we can properly provide binary kmod packages.

  ./configure --with-config=srpm
  make SRPM_DEFINE_KMOD='--define="repo rpmfusion"' srpm-kmod
  mock rebuild zfs-kmod-x.y.z-r.el6.src.rpm

One nice benefit of finishing this work is that the generic and
fedora spl-kmod spec files can be merged again.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>