Right now there is some funky logic to either use a previous build as a
seed or the current SDK tarball if it happens to have been downloaded.
This is a bit confusing and doesn't work reliably since it is reasonable
for there to be neither a previous build or the current SDK available if
the SDK chroot was created some time ago. Fix this by using the new SDK
library and always use the latest SDK, downloading it if needed.
Currently we don't have a good way to upload packages from different
jobs to the same location. The 'Packages' index file is only generated
locally so the second upload would always win.
This replaces the cross-toolchain compile step in bootstrap_sdk and adds the
ability to build native toolchains using the cross toolchain. This is just
the first step towards actually providing the native toolchain in a container.
