Merge pull request #6 from marineam/toolchain

fix(cros_setup_toolchains): Don't default to the gold linker.
fix(cros_sdk): Use SDK version from version.txt in the manifest repo

Second stage of moving version information from coreos-overlay to the
manifest.
clean(upload_prebuilts): Our upload code is in the scripts repo

Removing this removes the last reference to
coreos-overlay/coreos/binhost which I would also like to delete.
Merge pull request #7 from marineam/sdk-version

Sdk version
fix(cros_setup_toolchains): Always install packages when using crossdev

The latest SDK tarballs include the cross toolchain but the build
process depends on the cross toolchain binary packages to be present
which they won't be if they didn't need to be upgrade/installed.

To avoid the situation force each package to be downloaded re-installed
when initializing a new chroot via crossdev instead of no-opping when
they are already installed. Future SDK builds will be updated to not
include the toolchain as before since it is just needlessly increasing
the SDK tarball size.