{kernel, module}.bbclass: don't run depmod for module packages during do_rootfs

* depmod already gets executed by pkg_postinst_kernel-image.

* If you build a module using module.bbclass, pkg_postinst returns 1 in
  do_rootfs, causing pkg_postinst to run again on first boot. To improve
  this situation, I copied pkg_postinst from kernel.bbclass to module.bbclass.
  This was rejected by Koen, because he doesn't like the code from
  kernel.bblcass, which uses ${STAGING_DIR_KERNEL}. Richard then suggested
  that calling depmod during do_rootfs wasn't necessary at all, because
  it already gets done by kernel-image.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>