This way all users of storage can share the logic to choose between flash or RAM
storage depending on the "std" feature. This is needed because the store_latency
example assumes flash storage but is built when running `cargo test
--features=std`.
We don't need to build a production key without persistent storage. Tests and
fuzzing continue to use the std feature to use the RAM implementation (that does
sanity checks).
This permits to run without persistent storage. The benefit is that the board
doesn't implement a the syscall API in Tock. The disadvantage is that rebooting
the key will reset the storage.
