afb-export: Manage path of libraries

This change allows a binding to know its
installation path. This path is retrieved using
the api function 'afb_api_settings'.
The path is returned --if known-- as a string
of key "binding-path".

Change-Id: Ie1a349dc4936d4ccf173f1f77e118099f7f0599a
Signed-off-by: José Bollo <jose.bollo@iot.bzh>