nixos: introduce system.nixosLabel option and use it where appropriate

Setting nixosVersion to something custom is useful for meaningful GRUB
menus and /nix/store paths, but actuallly changing it rebulids the
whole system path (because of `nixos-version` script and manual
pages). Also, changing it is not a particularly good idea because you
can then be differentitated from other NixOS users by a lot of
programs that read /etc/os-release.

This patch introduces an alternative option that does all you want
from nixosVersion, but rebuilds only the very top system level and
/etc while using your label in the names of system /nix/store paths,
GRUB and other boot loaders' menus, getty greetings and so on.

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

@@ -83,7 +83,7 @@ addEntry() {
     timestampEpoch=$(stat -L -c '%Z' $path)
 
     timestamp=$(date "+%Y-%m-%d %H:%M" -d @$timestampEpoch)
-    nixosVersion="$(cat $path/nixos-version)"
+    nixosLabel="$(cat $path/nixos-version)"
     extraParams="$(cat $path/kernel-params)"
 
     echo
@@ -91,7 +91,7 @@ addEntry() {
     if [ "$tag" = "default" ]; then
         echo "  MENU LABEL NixOS - Default"
     else
-        echo "  MENU LABEL NixOS - Configuration $tag ($timestamp - $nixosVersion)"
+        echo "  MENU LABEL NixOS - Configuration $tag ($timestamp - $nixosLabel)"
     fi
     echo "  LINUX ../nixos/$(basename $kernel)"
     echo "  INITRD ../nixos/$(basename $initrd)"