Add NVStore (A.K.A SOTP) feature

Author: David Saada

features/nvstore/README.md

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+# NVStore
+
+A lightweight module providing the functionality of storing data by keys in the internal flash, for security purpose.
+
+## Description
+
+NVStore provides the ability to store a minimal set of system critical items in the internal flash.
+For each item key, NVStore module provides the ability to set the item data or get it.
+Newly set values are added to the flash (as in journal), with the effect of overriding the previous value for this key.
+NVStore module ensures that existing data isn't harmed by power failures, during any operation.
+The full interface can be found under nvstore.h.
+
+### Flash structure
+NVStore uses two Flash areas, active and non-active. Data is written to the active area, until it gets full.
+When it does, garbage collection is invoked, compacting items from the active area to the non-active one,
+and switching activity between areas.
+Each item is kept in an entry, containing header and data, where the header holds the item key, size and CRC.
+Each area must consist of one or more erasable units (sectors). 
+
+### 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
+
+### Configuring NVStore for your board
+NVStore requires the addresses and sizes of both areas in flash. For this purpose, the following values should be defined in 
+configuration:
+NVSTORE_AREA_1_ADDRESS
+NVSTORE_AREA_1_SIZE 
+NVSTORE_AREA_2_ADDRESS
+NVSTORE_AREA_2_SIZE
+This is currently supported by the K64F and K82F boards, while these values are configured in the targets.json file. Any new board
+supporting this feature should have these values added there. 
+
+### Building NVStore
+The default value of number of different keys can be set by the NVSTORE_MAX_KEYS define in mbed_lib.json .
+
+### Using NVStore
+NVStore is a singleton class, meaning that the system can have only a single instance of it.
+To instanciate NVStore, one needs to call its get_instance member function as following:
+    NVStore &nvstore = NVStore::get_instance();
+After the NVStore instantiation, one can call the init API, but it is not necessary, as all
+NVStore APIs (get, set et al.) 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