Now this code can be shared with setup_board. Only required if
setup_board is called with --nousepkg which is rare to never but feels
like the correct thing to do. Alternatively setup_board could always
use binary packages (as it basically does now).
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.
