mesh: Save newly added or updated net key to config file

mesh: Save newly added or updated net key to config file

This separates mesh_db_net_key_add() into distinct functions:
mesh_db_net_key_add() and mesh_db_net_key_update() which will be called
based on whether a network key was newly added or updated.

Change-Id: I7af60c622c39f813a5aab64b5ae936943a1cfcb8
Signed-off-by: Anupam Roy <anupam.r@samsung.com>