He was definitely seeding life. This has been stated clearly by Ridley himself and is quite obvious given that the planet appears to be in its formative stages with only minimal plant life.

A pre-vis on the blu-ray actually shows a very primitive amphibian creature crawl out of the water and onto land after the DNA sequence.  

Our oldest ancestor, LOL.

My personal belief is that this is NOT the BLACK GOO, this was probably millions of years ago, and the effects are completely different than those of the BLACK GOO. It also looks different.

Another scene on the blu-ray shows Ridley designing the SACRIFICIAL GOO with various berries and coloured honeycomb mashed into a bowl. If it was the BLACK GOO, he would have simply made it BLACK!!! ITS NOT EVEN BLACK PEOPLE!! 

- Phase one: create people
- Phase two: force evolutionary change with Xenomorph DNA or "BLACK GOO"

The only question remaining about Prometheus (originally titled Alien Engineers) is why they would want Xenomorphs. Weapon? Cleansing a planet like a Petri dish?