dracut: add --kernel-only and --no-kernel arguments

dracut: add --kernel-only and --no-kernel arguments

       --kernel-only
              only install kernel drivers and firmware files

       --no-kernel
              do not install kernel drivers and firmware files

All kernel module related install commands moved from "install"
to "installkernel".

For "--kernel-only" all installkernel scripts of the specified
modules are used, regardless of any checks, so that all modules
which might be needed by any dracut generic image are in.

The basic idea is to create two images. One image with the kernel
modules and one without. So if the kernel changes, you only have
to replace one image.

Grub and the kernel can handle multiple images, so grub entry can
look like this:

title Fedora (2.6.29.5-191.fc11.i586)
        root (hd0,0)
        kernel /vmlinuz-2.6.29.5-191.fc11.i586 ro rhgb quiet
        initrd /initrd-20090722.img
        initrd /initrd-kernel-2.6.29.5-191.fc11.i586.img
        initrd /initrd-config.img

initrd-20090722.img
  the image provided by the initrd rpm
  one old backup version is kept like with the kernel

initrd-kernel-2.6.29.5-191.fc11.i586.img
  the image provided by the kernel rpm

initrd-config.img
  optional image with local configuration files