nvs: Support for handling multiple NVS partitions

nvs: Support for handling multiple NVS partitions

This commit adds support for multiple NVS partitions. This provides application a flexibility to have multiple NVS
partitions such as separate partition with read-only manufacturing data and read-write partition with configuration.
Application can also use this to separate out application's configuration storage from system configuration.

This feature does not change any of the basic property of NVS subsystem. The same-named namespaces across partitions are
considered to be different namespaces. The original NVS API available for the applications remains unchanged. The only
difference is that instead of first NVS partition in the partition table, it now operates on the partition with label
"nvs" (which is default in the IDF provided partition table files). Additional APIs are provided to open a handle and
erase NVS with partition name as a parameter.

A test case is added in the host tests and it is made sure that all the host tests pass. nvs_rw_value app is also tested
with multiple partitions.

Signed-off-by: Amey Inamdar <amey.inamdar@gmail.com>