os-helpers-fs: add function to erase disks

[alexgg]

The function overwrites the first 16 MiB of each partition (the default LUKS2 header size), as well as the primary and backup GPT partition tables.

It's typically called from the cryptsetup flasher initramfs script when running on a locked device to close attack vectors that use the flasher images on locked devices, for example in balena-os/meta-balena-hab@e638c64

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[jakogut]

Looks fine to me