Bug 1087456 part1: Update base image to CentOS 6.

releng uses CentOS 6 for CI. In order to reuse their packages, we also
move from Ubuntu to CentOS 6 for our base images.
Bug 1087456 part2: change builder image from "gecko-base" to "base".

builder image is now based on the "base" image that lives on gecko
tree.
Bug 1087456 part3: create repositories parent dirs if they don't exist.

Older versions of mercurial cannot create inner directories if the
parents don't exist.
Bug 1087456: use build-setup vars to reference repos.
Bug 1087456: symbolic link gaia repo inside gecko tree.

desktop nightly mozconfig [1] expects to find gaia tree
inside gecko tree.

[1] http://hg.mozilla.org/mozilla-central/file/tip/b2g/config/mozconfigs/linux64_gecko/nightly