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

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>