add(bootstrap_sdk): Add a script for bootstrapping the sdk tarball

This uses Gentoo's catalyst for very thoroughly building images from
scratch. Using images based on this will eliminate some of the hackery
in make_chroot.sh for building up the sdk from a stock stage3 tarball.

For reference the procedure it performs is this:

1. snapshot: Grab a snapshot of portage-stable. Note that overalys are
   not snapshotted.
2. stage1: Using a "seed" tarball as a build environment, build a
   minimal root file system into a clean directory using ROOT=...
   and USE=-* The restricted USE flags are key be small and avoid
   circular dependencies.
3. stage2: Run portage-stable/scripts/bootstrap.sh
   This rebuilds the toolchain. Probably not strictly necessary most of
   the time but does super-duper-promise that the toolchain isn't linked
   to or otherwise influenced by whatever was in the "seed" tarball.
4. stage3: Run emerge -e system to rebuild everything using the fresh
   toolchain using the normal USE flags provided by the profile. This
   will also pull in assorted base system packages that weren't included
   in the minimal environment stage1 created.
5. stage4: Install any extra packages or other desired tweaks. For the
   sdk we just install all the packages normally make_chroot.sh does.
fix(catalyst): Add --with-bdeps=y to stage4 upgrade

By default emerge will not install build dependencies if it doesn't need
them (i.e. installing a binary package) but we want to make sure
everything gets included in stage4 no matter how it was installed.
fix(make_chroot): Remove unnecessary package rebuilds/removals

These are not required when starting from the new sdk stage4 tarballs
instead of a stock gentoo stage3 tarball.
Merge pull request #5 from marineam/bootstrap

Bootstrap