build omnibus packages in tmp-dest

This is a breaking change! However, it improves consistency between
omnibus and regualar recipes. Regular recipes usually contain the
following:

    def build
      configure :prefix => prefix
      make
    end
    def install
      make 'DESTDIR' => destdir
    end

Omnibus recipes on the other hand looks like this:

    def build
      configure :prefix => destdir
      make
    end
    def install
      make
    end

With this patch, both can use the first form. Moreover building omnibus
packages without write privileges in /opt and packing omnibus fpm with omnibus fpm is now possible.

The next step would be repairing the examples and removing the omnibus_installing directive which is obsolete with this patch.

Cheers'
Hannes