mesh: Separate functions for net key add and update

mesh: Separate functions for net key add and update

This splits mesh_net_key_add() into two separate functions:
mesh_net_key_add() and mesh_net_key_update().
mesh_net_key_update() essentially replaces mesh_net_kr_phase_one()
since switching to Key Refresh phase one can only be triggered
by successful network key update.

Change-Id: I9be2f9784ebb094906107bd8923694199e0900f3
Signed-off-by: Anupam Roy <anupam.r@samsung.com>