Make the directory where pmaports resides a configurable option

It would be nice if we could set the pmaports symlink location as a configurable option (so it must not be placed in the cache_git directory). Setting this makes it possible to point it to an existing repository clone of pmaports while not having to give the --aports/-p parameter with every command.

We should be able to set the option like this: pmbootstrap config aport /some/other/directory/