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.

README.rst

@@ -118,6 +118,19 @@ Optionally install archiso's mkinitcpio hooks:
 
     make install-initcpio
 
+Optional Features
+=================
+
+The iso image contains a grub environment block holding the iso version. This allows to boot the iso image from grub
+with a version specific cow directory to mitigate overlay clashes.
+
+  .. code:: grub
+      loopback loop archlinux.iso
+      load_env -f (loop)/arch/grubenv
+      linux (loop)/arch/boot/x86_64/vmlinuz-linux ... \
+          cow_directory=archlinux/${VERSION} ...
+      initrd (loop)/arch/boot/x86_64/initramfs-linux-lts.img
+
 Contribute
 ==========
 

archiso/mkarchiso

@@ -834,11 +834,19 @@ _set_overrides() {
     _msg_info "Done!"
 }
 
-
 _export_gpg_publickey() {
     gpg --batch --output "${work_dir}/pubkey.gpg" --export "${gpg_key}"
 }
 
+_make_version() {
+    install -d -m 0755 -- "${isofs_dir}/${install_dir}"
+    _msg_info "Creating files with iso version..."
+    printf '%s\n' "${iso_version}" > "${airootfs_dir}/version"
+    printf '%s\n' "${iso_version}" > "${isofs_dir}/${install_dir}/version"
+    printf '%.1024s' "$(printf '# GRUB Environment Block\nVERSION=%s\n%s' "${iso_version}" \
+        "$(printf '%0.1s' "#"{1..1024})")" > "${isofs_dir}/${install_dir}/grubenv"
+    _msg_info "Done!"
+}
 
 _make_pkglist() {
     install -d -m 0755 -- "${isofs_dir}/${install_dir}"
@@ -867,6 +875,7 @@ _build_profile() {
     [[ -n "${gpg_key}" ]] && _run_once _export_gpg_publickey
     _run_once _make_custom_airootfs
     _run_once _make_packages
+    _run_once _make_version
     _run_once _make_customize_airootfs
     _run_once _make_pkglist
     _make_bootmodes