fix(release_util): Move default upload root to common, add --upload_root

Make it possible for other scripts to share the same value for our
release repository and equally easy to override with a custom value.
Also allow setting the root from the command line in addition to the
environment. Usually --upload_root is better to use than --upload_path.
fix(common): Define VERSION_ID and BUILD_ID

Scripts can now use COREOS_VERSION_ID to get the base version instead of
having to strip off the BUILD_ID that is appended for unofficial builds.
feat(setup_board): Configure binary package sources on the fly.

Generate list of board binhosts based on the new $COREOS_DOWNLOAD_ROOT
variable and optionally use packages only from a specific version
fix(setup_board): Remove note about annoying eselect warning

Silenced in eselect 1.4
Merge pull request #169 from marineam/binhost

binhost config update