Cross bootstrap examples

A brief overview of this process: the goal is to native build as much of the system as possible because that's easier, but in order to do so we need at least 'build-essential'. 'morph cross-bootstrap' will cross-build any bootstrap-mode chunks in the given system and will then prepare a large rootfs tarball which, when booted, will build the rest of the chunks in the system using the cross-built build-essential.

This approach saves us from having to run Morph, Git, Python, Perl, or anything else complex and difficult to cross-build on the target until it is bootstrapped. The user of this command needs to provide a kernel and handle booting the system themselves (the generated tarball contains a /bin/sh that can be used as 'init').