Merge pull request #5900 from davidsaada/david_sotp_cpp

Add NVStore (A.K.A SOTP) feature

Author: cmonr

features/nvstore/README.md

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+# NVStore
+
+NVStore is a lightweight module that stores data by keys in the internal flash for security purposes. 
+
+## Description
+
+NVStore provides the ability to store a minimal set of system critical items in the internal flash.
+For each item key, the NVStore module provides the ability to set the item data or get it.
+Newly added values are added to the end of the existing data, superseding the previous value that was there for the same key. 
+The NVStore module ensures that power failures don't harm existing data during any operation.
+The full interface can be found under `nvstore.h`.
+
+### Flash structure
+NVStore uses two Flash areas, active and nonactive. Each area must consist of at least one erasable unit (sector).
+Data is written to the active area until it becomes full. When it does, garbage collection is invoked.
+This compacts items from the active area to the nonactive one and switches activity between areas.
+Each item is kept in an entry containing a header and data, where the header holds the item key, size and CRC.
+
+### APIs
+- init: Initialize NVStore (also lazily called by get, set, set_once and remove APIs).
+- deinit: Deinitialize NVStore.
+- get: Get the value of an item, given key.
+- set: Set the value of an item, given key and value.
+- set_once: Like set, but allows only a one time setting of this item (and disables deleting of this item).
+- remove: Remove an item, given key.
+- get_item_size: Get the item value size (in bytes).
+- set_max_keys: Set maximal value of unique keys. Overriding the default of NVSTORE_MAX_KEYS. This affects RAM consumption,
+  as NVStore consumes 4 bytes per unique key. Reinitializes the module.
+
+
+## Usage
+
+### Enabling NVStore and configuring it for your board
+NVStore is enabled by default for all devices with the internal flash driver (have "FLASH" in the device_has attribute).
+One can disable it by setting its "enabled" attribute to false.
+Unless specifically configured by the user, NVStore selects the last two flash sectors as its areas, with the mininum size of 4KBs,
+meaning that if the sectors are smaller, few continuous ones will be used for each area.
+The user can override this by setting the addresses and sizes of both areas in` mbed_lib.json` on a per board basis.
+In this case, all following four attributes need to be set:
+- area_1_address
+- area_1_size
+- area_2_address
+- area_2_size
+
+In addition, the `num_keys` value should be modified to change the default number of different keys.
+
+### Using NVStore
+NVStore is a singleton class, meaning that the system can have only a single instance of it.
+To instantiate NVStore, one needs to call its get_instance member function as following:
+``` c++
+    NVStore &nvstore = NVStore::get_instance();
+```
+After the NVStore instantiation, one can call the init API, but it is not necessary because all
+NVStore APIs (get, set and so on) perform a "lazy initialization".
+
+### Testing NVStore
+Run the NVStore functionality test with the `mbed` command as following:
+```mbed test -n features-nvstore-tests-nvstore-functionality```