dracut: Don't fail at copying files when including directories

dracut: Don't fail at copying files when including directories

When including a directory, the files were considered in the directory
name which lead to messages like :

cp: failed to access '/var/tmp/initramfs.L9s2zO///init-func': No such file or directory

This patch does make the destdir more explicit and copy files into the
destination directory instead of destdir/filename/