mcf: Fix parsing more than 2 layers with same priority

* with 2 or more layers with same priority, using xlayer is not enough to
  maintain list of processed layers
* adding meta-oe, meta-ruby and meta-networking with same priority 6 added
  only meta-networking and twice meta-oe (meta-ruby was removed from
  bblayers.conf after adding meta-networking)

[GF-2143] Add meta-ruby layer
[GF-2650] Add meta-networking layer