update to match modern layout and use github repos

Even with this added, it does not work too well. Github does not allow any hierachical structure for repos, so with the relative submodule configuration, the raspberry-pi manifest will use plain/default core and base. That will not work for the initial checkout, since it is referencing commits only present in oe-lite rpi core/base.

We can consider multiple solutions:

1 - create "core-rpi" and "base-rpi" and reference these. Doing this we of course end up with _a lot_ of repos in the oe-lite org at some point.
2 - create an "oe-lite-rpi" organisation. this allows us to have completely seperate repos for a manifest like the pi one. See: http://stackoverflow.com/questions/11852982/arrange-repositories-in-folders-on-github
3 - work with plain meta/foo repos. Should be possible over time?
4 - start using branches on meta/foo repos for manifest specific additions. probably not nice.

/Kim