tools/mesh-cfgclient: Command line option for config

tools/mesh-cfgclient: Command line option for config

This adds "-c" option to specify full path to configuration file.
If the path is not provided, the application will attempt to open
in the following order:
1) $XDG_CONFIG_HOME/meshcfg/config_db.json
2) $HOME/meshcfg/config_db.json

if the file does not exist, it will be generated for a newly created
network, i.e., upon successful completion of "create" command.

Change-Id: Ica273fe3803a09ab8d54434dbbcb586c0620f16a
Signed-off-by: Anupam Roy <anupam.r@samsung.com>