weston: Create really shared files

Last time it was discussed, this patch was not upstreamable.
But it is needed for a reason that still have to be investigated.
Nevertheless the patch is enough simple to be maintained
while waiting for the full investigation/resolution.

Change-Id: I47b17c7aa9ca5c495267259a15caacef50b4b324
Signed-off-by: José Bollo <jose.bollo@iot.bzh>