provision: add script for managing device provisioning data

provision: add script for managing device provisioning data

This is useful for keeping specific data on a device across factory reset.
It uses a separate partition (only UBI supported at the moment) to store
the data. The primary use case is storing sensitive data like cryptographic
keys for maintaining a device as part of a network.

Signed-off-by: Felix Fietkau <[email protected]>