Function now always rewrites old value under same key
regardless existing data type. Users requiring old API behaviour
can enable it by kconfig option CONFIG_NVS_LEGACY_DUP_KEYS_COMPATIBILITY
* Move nvs flash README to common doc directory
* correct markup of functions and types in text
from old README
* Better comment of nvs_get_used_entry_count()
* Mention C++ example in API reference
* Used target instead of hard code ESP32
* Note that strings can only span one page
* Reflect that item types have been moved
* Some clarification about nvs_commit()
* Improved reference to the ESP Partition API
* fixed little mistake in documenting-code.rst
* Change of nvs_open_from_part() to
nvs_open_from_partition() reflected in docs
* Corrected documentation of
NVSHandle::get_string(), NVSHandle::get_blob()
and NVSHandle::get_item_size().
* Closes DOC-165
* Closes IDF-1563
* Closes IDF-859
* Closes https://github.com/espressif/esp-idf/issues/6123
