Add configuration file over-ride mechanism

Reusing the windowmanager on the cluster demo platform requires
modifications to the layers.json, areas.json, 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.

Cherry-picked from flounder branch with rework to accommodate changes
in master branch.

Bug-AGL: SPEC-1805

Change-Id: Ib0014756fe684167950ddac534ea2363a1b6e7c3
Signed-off-by: Scott Murray <scott.murray@konsulko.com>