ostree-image-uefi: disable systemd-gpt-auto-generator

Added kernel option `systemd.gpt_auto=false` to explicitly
disable `systemd-gpt-auto-generator`.
With a bind-mount of `/boot` partition the GPT generator creates the
automount for EFI partition targeted to `/bind` and fail during the boot.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>