mesh: Replace storage_save_config with mesh_config_save_config

mesh: Replace storage_save_config with mesh_config_save_config

This moves writing out of node configuration from storage.c down to
mesh-config-<format-specific>.c to allow for more generic storage layout.

New generalized API in mesh-config.h:
mesh_config_save_config(struct mesh_config *cfg, bool no_wait,
				mesh_config_status_func_t cb, void *user_data)

replaces the old one in storage.h:
storage_save_config(struct mesh_node *node, bool no_wait,
					mesh_status_func_t cb, void *user_data)

Currently, only JSON format is supported for storing node configuration:
mesh_config_save_config is implemented in mesh-config-json.c

Change-Id: I6ecb77332911bdb6a6ae18feedb8cae40b2832ff
Signed-off-by: Anupam Roy <anupam.r@samsung.com>