etlgraph and mapping-graph generated only in one directory

[frodrigo]

While working on supporting addons layers for OpenMapTiles openmaptiles/openmaptiles#970 I found issue on graph generation.

The addon layers are not in the same directory as the main layers. But generate-etlgraph and generate-mapping-graph output are not relative to tileset definition.

I understand why it was done like this, build from consolidated tileset definition, test. But need to make it more relative.