oe-setup-layers: make "path" optional

oe-setup-layers: make "path" optional

Layer names and path are often redundant. Allow users to omit the path
key when it is equal to the layer name by using the layer name as a
default value for its path.

For example, from layers.example.json:
  "sources": {
      "meta-alex": {
          "path": "meta-alex"
      },
      "meta-intel": {
          "path": "meta-intel"
      },
      "poky": {
          "path": "poky"
      }
  },

Update the schema to stop requiring "path" and remove it in example for
"meta-intel".

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>