Refactor KVStore config override explanation ...

The documentation previously referred to a weakly defined function
`storage_configuration`, however, this function was replaced at some
stage by `kv_init_storage_config`. Refactor the explanation on how to
override the default configurations to reflect this. Also, remove
the snippet which was used to show the implentation of
`storage_configuration`.

Deprecate TDB_EXTERNAL_NO_RBP and FILESYSTEM_NO_RBP in kv_config ...

NO_RBP (no rollback protection) is intended to not require an internal
TDB, however, DeviceKey, which we use to derive SecureStore's
encryption key, still does. Currently, no internal TDB is created with
these two configurations, meaning there's no way to store the DeviceKey
and SecureStore doesn't work.

BlockDevice design doc: TDBStore no longer requires flash simulation

TDBStore design doc: flash simulation no longer required

Refactor storage docs