This syscall does the same as `delete` for a key.
The only difference is that it is designed to allow re-import of the key
if it was exported with `WrapKey`.

With the core trussed backend this does not change anything, as nothing is needed to
allow the re-import. However the same mechansim in the SE050 work differently, and
being able to differenciate permanent deletion and deletion with intent to re-import is crucial.

Similar to 5af424e, this patch updates
the `keystore` method to be able to create a keystore with a custom path.
This can be useful for backends that want to have a keystore distinct from
the core one.