bitbake: toaster: orm Add a constant for the CustomImageRecipe's layer name

Use a constant to define the name for the toaster custom images layer;
this constant is then used to identify this layer in various places.

(Bitbake rev: 2540969ec71612af7f9041cadcc401513e9b357b)

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/orm/models.py

@@ -1449,6 +1449,10 @@ class ProjectLayer(models.Model):
         unique_together = (("project", "layercommit"),)
 
 class CustomImageRecipe(Recipe):
+
+    # CustomImageRecipe's belong to layers called:
+    LAYER_NAME = "toaster-custom-images"
+
     search_allowed_fields = ['name']
     base_recipe = models.ForeignKey(Recipe, related_name='based_on_recipe')
     project = models.ForeignKey(Project)