toaster: add toaster layer configuration files

In managed mode, Toaster creates project configurations
based on the layers that are being checked-out on disk.

The defaults are set through a JSON configuration
file that contains layer-specific specific information.

This patch adds toasterconf.json files for the oe core layer,
that make Toaster work with a standalone OpenEmbedded-Core checkout.

The toasterconf.json files describe basic configuration for Toaster
projects, including default layers, default configuration variable values,
toaster-compatible branches, and the layerindex URL
where the base layer and compatible layers.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>