mkarchiso: add version information

To date the iso version was used for iso volume information and iso file name.
In my custom builds I do use it a lot more:

* Inside the root fs: The system knows about its own version. I use this to:
  -> report the version to a server (poor man's inventory)
  -> let the system update itself

* On the iso fs: The files are served via rsync, running systems transfer
  version file first to check for available update.

* A grub environment file on the iso fs: Booting the iso from grub allows
  to create cow directory per version:

    loopback loop archlinux.iso
    load_env -f (loop)/arch/grubenv
    linux (loop)/arch/boot/x86_64/vmlinuz-linux ... \
        cow_directory=archlinux/${VERSION} ...

So let's just create these files.

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