orpsoc: add fetch command

Sometimes you may want to fetch a single core (and its dependencies
in your cache directory. This patch adds a command to do this.

If you want to see your cache directory in your build dir, add
cache_root = ./cache in your orpsoc.conf.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Merge pull request #14 from fjullien/fetch

orpsoc: add fetch command