libbuxton: Adding list_names feature

The feature (buxton_)list_names allows to fetch
names of buxton for the purpose of administrating.

The current implementation does not list the layers.

Change-Id: I4de72e47fbd3af6d3489817741890ef343d72a9a
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>