Makefile: rename cross target -> toolchain

- Use a more descriptive name, the same of the "toolchain" directory.
- Add missing dependencies to be able to successfully use the target
  right after the configuration.
- Move to a better position.
- Documentation it in the help target.
- Use toolchain target in the world target

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Markos Chandras <markos.chandras at imgtec.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout at mind.be>
Acked-by: Samuel Martin <s.martin49 at gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>