mgmt: mcumgr: grp: settings_mgmt: Allow saving single setting

Adds functionality that allows saving a single setting to NVM using
the newly added function in the settings subsystem. This also
replaces calling the subtree save function if it the underlying
Kconfig is enabled to reduce code paths in settings mgmt

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
2 files changed