initramfs-init: add a disable feature to tiny-cloud

Since Alpine Linux 3.18, it has a new feature "Experimental support for unattended installs via tiny-cloud".

Some VPS providers provide cloud-init files (meta-data, user-data) on /dev/sr0, which overwrite existing .apkovl.tar.gz configuration.

Creating a blank (/etc/tiny-cloud-init.disabled) file in the .apkovl.tar.gz ensures that tiny-cloud is not loaded.

Ref: https://cloudinit.readthedocs.io/en/latest/howto/disable_cloud_init.html