Replace dracut unified kernel image kernel-install script with objcopy

Replace dracut unified kernel image kernel-install script with objcopy

When using objcopy, we can generate the initrd in the install phase
which is cached which speeds up builds of incremental images. We use
objcopy from the host system to avoid having to install binutils in
all generated images.

We also call kernel-install manually in opensuse, debian and ubuntu
builds to make sure we can find an initrd image to pass to objcopy.
By calling kernel-install, we also make sure that
--without-unified-kernel-images now works for these distros as well.