Draft: boot-state-partition: Add partition backend for nvidia cboot

Add a new backend for a modified version of cboot that can perform boot counting in a spare partition.

This proof of concept assumes the partition is the 42nd partition on the primary storage eMMC. If the device hasn't been set up in this way, a magic check should protect the contents of that partition.

Signed-off-by: Derek Foreman derek.foreman@collabora.com