Improve privacy mode handling

Currently stack defaults to device privacy mode and allows applications to override this with ble_gap_set_priv_mode().
This means that application willing to use network privacy mode has to update mode after pairing and after bond information is loaded from storage.

This to improve:
- provide mynewt val that would allow to select default privacy mode
- persistently store privacy mode with rest of bonding information

Start with ble_gap_set_priv_mode(), then trace where pairing and bond information are loaded and stored. Review how Mynewt configuration values are defined to locate the proposed default privacy-mode setting. Done means applications can select the default mode and the selected privacy mode persists with the bonding information.

Written by the indexing model from the issue text.