fix(cros-workon.eclass): Add support for projects in src/platform

We no longer have any correlation between ebuild category and whether
the local checkout can be found in src/platform or src/third_party.
Instead provide a new variable to manually specify which it should be.
As-is it isn't possible to build local changes in src/platform trees.
bump(coreos-base/cros-devutils): update to latest code

This pulls in recent changes to cros_generate_update_payload
bump(coreos-base/update_engine): Update stable release to latest code

The stable ebuild hasn't been bumped to include quote a lot of recent
work yet so lets do that. :)
Merge pull request #59 from marineam/updates

coreos-base/update_engine + cros-workon.eclass