esp32: Add basic support for Non-Volatile-Storage in esp32 module.

This commit implements basic NVS support for the esp32.  It follows the
pattern of the esp32.Partition class and exposes an NVS object per NVS
namespace.  The initial support provided is only for signed 32-bit integers
and binary blobs.  It's easy (albeit a bit tedious) to add support for
more types.

See discussions in: #4436, #4707, #6780

docs/library/esp32.rst

@@ -269,3 +269,51 @@ Constants
           esp32.WAKEUP_ANY_HIGH
 
    Selects the wake level for pins.
+
+Non-Volatile Storage
+--------------------
+
+This class gives access to the Non-Volatile storage managed by ESP-IDF. The NVS is partitioned
+into namespaces and each namespace contains typed key-value pairs. The keys are strings and the
+values may be various integer types, strings, and binary blobs. The driver currently only
+supports 32-bit signed integers and blobs.
+
+.. warning::
+
+    Changes to NVS need to be committed to flash by calling the commit method. Failure
+    to call commit results in changes being lost at the next reset.
+
+.. class:: NVS(namespace)
+
+    Create an object providing access to a namespace (which is automatically created if not
+    present).
+
+.. method:: NVS.set_i32(key, value)
+
+    Sets a 32-bit signed integer value for the specified key. Remember to call *commit*!
+
+.. method:: NVS.get_i32(key)
+
+    Returns the signed integer value for the specified key. Raises an OSError if the key does not
+    exist or has a different type.
+
+.. method:: NVS.set_blob(key, value)
+
+    Sets a binary blob value for the specified key. The value passed in must support the buffer
+    protocol, e.g. bytes, bytearray, str. (Note that esp-idf distinguishes blobs and strings, this
+    method always writes a blob even if a string is passed in as value.)
+    Remember to call *commit*!
+
+.. method:: NVS.get_blob(key, buffer)
+
+    Reads the value of the blob for the specified key into the buffer, which must be a bytearray.
+    Returns the actual length read. Raises an OSError if the key does not exist, has a different
+    type, or if the buffer is too small.
+
+.. method:: NVS.erase_key(key)
+
+    Erases a key-value pair.
+
+.. method:: NVS.commit()
+
+    Commits changes made by *set_xxx* methods to flash.