add(coreos-devel/coreos-buildbot): Add ebuild for our buildbot tools.

Creating a new category for this, coreos-base is overused and dev-python
didn't seem right for custom infrastructure tools. Going forward I'd
like to put SDK and build host specific stuff in this category when
previously coreos-base would be used. Things that actually land in
images would stick with coreos-base.
add(coreos-devel/*-depends): Meta ebuilds for buildhost and SDK.

Create a meta-ebuild for the SDK based on the packages currently listed
explicitly in bootstrap_sdk.sh and a buildhost ebuild that expands on
that, adding packages that are required in containers used by build
slave instances.
fix(profiles): Set use flags required by buildhost-depends
fix(coreos-devel/coreos-buildbot): Update git commit hash

Merged, use fresh new merge hash.
Merge pull request #143 from marineam/buildbot

Buildhost ebuilds