FIXUP: build: abstract CrossCompile type logic

* Make CrossCompile a proper enum type rather than a string literal,
* Introduce methods to get the correct host/build chroots depending on the
  cross compile type and target architecture.
* Remove autodetect.chroot() since it doesn't do what we expect, adjust
  all users to use cross.build_chroot() instead.
* Refactor package building to correctly use cross.host_chroot() and
  cross.build_chroot().

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: !2568