Add the allocate_key API. This replaces the previously added set_alloc_key API
(which allocates a key and sets the value at the same time).
Reason for the change: Key allocation will typically be used by other storage
features (like StorageLite), keeping the allocated keys in another location.
Previous API created problems in the case key allocation and value setting
couldn't be done at the same time (for instance, if the set value was
derived from the allocated key, such as hash or CMAC).
- Define an enum for predefined keys (later filled by internal users of NVStore)
- Add the set_alloc_key API, allocating a free key from the non predefined keys