Add configuration file over-ride mechanism

Add configuration file over-ride mechanism

Reusing the windowmanager on the cluster demo platform requires
modifications to the layers.json, areas.db, etc. configuration files.
To allow this without breaking potential shared package feeds by
building two versions, add a mechanism to allow over-riding the
configuration files present in the widget with files in the directory
/etc/xdg/windowmanager.  Affected code has been refactored to use a new
common configuration file path determination helper function to reduce
duplication.

Bug-AGL: SPEC-1805

Change-Id: Ia3e1af0703794079f9537b934b124788129c045c
Signed-off-by: Scott Murray <scott.murray@konsulko.com>