As you control meta-webos, there's no need for the recipes to expect that META_WEBOS_LAYER has been set. This being a requirement makes it tricky to build webos outside of build-webos, as these variables also need to be set by hand.
META_WEBOS_LAYER can be removed seamlessly by simply setting it in meta-webos/conf/layer.conf:
META_WEBOS_LAYER := "${LAYERDIR}"
There's currently a single user of META_OE_LAYER. Arguably this could be added to meta-oe too, or the recipe restructured to avoid that need (for example, a clangifier BBCLASSEXTEND could work).