As previously said, there are three metadata groups—configuration, class, and recipe.
The first parsed metadata in BitBake is configuration metadata, identified by the .conf file extension. This metadata is global, and therefore, affects all recipes and tasks which are executed.
BitBake first searches the current working directory for an optional build/conf/bblayers.conf configuration file, and it is expected to contain a BBLAYERS variable that is a space-delimited list of layer directories. For each directory in this list, a build/conf/layer.conf file is searched for and parsed with the LAYERDIR variable being set to the directory where the layer was found. This process automatically sets up BBPATH and other variables for a given build directory for the user.
Tip
The order of the listed layers in the BBLAYERS variable is followed by BitBake when parsing the metadata. In case your layer needs to be parsed first, be sure to have it listed in the right order in BBLAYERS.
BitBake...